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Top 100 Senior Software Engineer Interview Questions and Answers

Top 100 Senior Software Engineer Interview Questions and Answers
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1. Explain the Singleton design pattern.

Answer:

The Singleton pattern ensures a class has only one instance and provides a global point of access to it. Below is a Java implementation:

public class Singleton {
    private static Singleton instance;

    private Singleton() {} // Private constructor prevents instantiation

    public static Singleton getInstance() {
        if(instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

Official Singleton Pattern Documentation


2. What is a deadlock in multithreading and how can it be prevented?

Answer:

A deadlock occurs when two or more threads are blocked forever, each waiting for the other to release a lock. To prevent deadlocks, use techniques like locking in a consistent order and setting timeouts for acquiring locks.

Official Java Concurrency Documentation


3. Explain the concept of Big O notation.

Answer:

Big O notation describes the upper bound of an algorithm’s time complexity. It helps analyze the efficiency and scalability of algorithms. For example, O(n) denotes linear time complexity.

Official Big O Notation Documentation


4. Write a function to reverse a linked list.

Answer:

public ListNode reverseLinkedList(ListNode head) {
    ListNode prev = null;
    ListNode current = head;
    while (current != null) {
        ListNode next = current.next;
        current.next = prev;
        prev = current;
        current = next;
    }
    return prev;
}

Official Linked List Documentation


5. Explain the principles of SOLID.

Answer:

SOLID is an acronym for five design principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. They guide software design and promote maintainability and flexibility.

Official SOLID Principles Documentation


6. What is the purpose of Dependency Injection?

Answer:

Dependency Injection is a design pattern that promotes loose coupling and modular design. It allows objects to receive their dependencies from an external source rather than creating them internally. This makes code more testable, maintainable, and flexible.

Official Dependency Injection Documentation


7. Write a function to find the Fibonacci sequence of a given number.

Answer:

public int fibonacci(int n) {
    if(n <= 1) {
        return n;
    }
    int prev1 = 0, prev2 = 1, fib = 0;
    for(int i = 2; i <= n; i++) {
        fib = prev1 + prev2;
        prev1 = prev2;
        prev2 = fib;
    }
    return fib;
}

Official Fibonacci Sequence Documentation


8. Explain the concept of Object-Relational Mapping (ORM).

Answer:

ORM is a programming technique that allows data to be manipulated and queried using an object-oriented paradigm, rather than raw SQL queries. It provides a bridge between the relational database and the application’s object model.

Official ORM Documentation


9. What are RESTful APIs and how do they work?

Answer:

RESTful APIs (Representational State Transfer) are a set of principles for designing networked applications. They use standard HTTP methods (GET, POST, PUT, DELETE) and represent resources as URLs. This allows for stateless, scalable, and uniform communication.

Official RESTful API Documentation


10. Write a function to find if a string is a palindrome.

Answer:

public boolean isPalindrome(String str) {
    int left = 0, right = str.length() - 1;
    while(left < right) {
        if(str.charAt(left++) != str.charAt(right--)) {
            return false;
        }
    }
    return true;
}

Official Palindrome Documentation


11. Explain the concept of Garbage Collection.

Answer:

Garbage Collection is an automatic memory management process in which the system automatically identifies and removes unused objects to free up memory. This prevents memory leaks and reduces the burden on the developer for manual memory management.

Official Garbage Collection Documentation


12. What is the purpose of Design Patterns in software development?

Answer:

Design Patterns are proven solutions to recurring design problems in software development. They provide a common language for developers to communicate solutions and promote best practices. They improve code readability, maintainability, and scalability.

Official Design Patterns Documentation


13. Write a function to find the longest common subsequence of two strings.

Answer:

public String longestCommonSubsequence(String str1, String str2) {
    int m = str1.length(), n = str2.length();
    int[][] dp = new int[m+1][n+1];
    for(int i = 1; i <= m; i++) {
        for(int j = 1; j <= n; j++) {
            if(str1.charAt(i-1) == str2.charAt(j-1)) {
                dp[i][j] = dp[i-1][j-1] + 1;
            } else {
                dp[i][j] = Math.max(dp[i-1][j], dp[i][j-1]);
            }
        }
    }
    int i = m, j = n;
    StringBuilder lcs = new StringBuilder();
    while(i > 0 && j > 0) {
        if(str1.charAt(i-1) == str2.charAt(j-1)) {
            lcs.insert(0, str1.charAt(i-1));
            i--;
            j--;
        } else if(dp[i-1][j] > dp[i][j-1]) {
            i--;
        } else {
            j--;
        }
    }
    return lcs.toString();
}

Official Longest Common Subsequence Documentation


14. Explain the concept of Multi-threading and when it is useful.

Answer:

Multi-threading allows a program to execute multiple threads concurrently. It’s useful when tasks can be performed independently and concurrently, such as in GUI applications, server handling multiple clients, and for background tasks.

Official Java Concurrency Documentation


15. Write a function to find all permutations of a string.

Answer:

public List<String> permutations(String str) {
    List<String> result = new ArrayList<>();
    permuteHelper(str, 0, result);
    return result;
}

private void permuteHelper(String str, int index, List<String> result) {
    if(index == str.length()) {
        result.add(str);
        return;
    }
    for(int i = index; i < str.length(); i++) {
        str = swap(str, i, index);
        permuteHelper(str, index + 1, result);
        str = swap(str, i, index);
    }
}

private String swap(String str, int i, int j) {
    char[] charArray = str.toCharArray();
    char temp = charArray[i];
    charArray[i] = charArray[j];
    charArray[j] = temp;
    return String.valueOf(charArray);
}

Official Permutations Documentation


16. Explain the concept of Inversion of Control (IoC).

Answer:

IoC is a design principle where control of the flow of a program is inverted. Instead of the programmer controlling the flow, the framework or container controls it. It’s commonly used in dependency injection.

Official IoC Documentation


17. What is the purpose of the Observer design pattern?

Answer:

The Observer pattern is used when one object (the subject) maintains a list of its dependents (observers) and notifies them of state changes, usually by calling one of their methods. This pattern is widely used for implementing distributed event handling systems.

Official Observer Pattern Documentation


18. Write a function to find the shortest path in a weighted graph using Dijkstra’s algorithm.

Answer:

public void dijkstra(Graph graph, Vertex source) {
    PriorityQueue<Vertex> pq = new PriorityQueue<>(Comparator.comparingInt(v -> v.distance));
    source.distance = 0;
    pq.add(source);

    while (!pq.isEmpty()) {
        Vertex u = pq.poll();

        for (Edge e : u.adjacents) {
            Vertex v = e.target;
            int weight = e.weight;

            if (u.distance + weight < v.distance) {
                v.distance = u.distance + weight;
                v.previous = u;
                pq.add(v);
            }
        }
    }
}

Official Dijkstra’s Algorithm Documentation


19. Explain the concept of Design by Contract.

Answer:

Design by Contract is a software development approach where the interactions between software components are defined by preconditions, postconditions, and invariants. It helps establish a clear understanding of the expected behavior of each component.

Official Design by Contract Documentation


20. Write a function to perform a binary search on a sorted array.

Answer:

public int binarySearch(int[] arr, int target) {
    int left = 0, right = arr.length - 1;

    while (left <= right) {
        int mid = left + (right - left) / 2;

        if (arr[mid] == target) {
            return mid;
        }

        if (arr[mid] < target) {
            left = mid + 1;
        } else {
            right = mid - 1;
        }
    }

    return -1;
}

Official Binary Search Documentation


21. Explain the concept of Memoization in dynamic programming.

Answer:

Memoization is a technique used to optimize recursive algorithms by storing the results of expensive function calls and returning the cached result when the same inputs occur again. It’s commonly used in dynamic programming.

Official Memoization Documentation


22. What is the purpose of the Flyweight design pattern?

Answer:

The Flyweight pattern is used to minimize memory usage and/or computational expenses by sharing as much as possible with related objects. It’s particularly useful when a large number of similar objects need to be created.

Official Flyweight Pattern Documentation


23. Write a function to implement a bubble sort algorithm.

Answer:

public void bubbleSort(int[] arr) {
    int n = arr.length;
    for (int i = 0; i < n-1; i++) {
        for (int j = 0; j < n-i-1; j++) {
            if (arr[j] > arr[j+1]) {
                int temp = arr[j];
                arr[j] = arr[j+1];
                arr[j+1] = temp;
            }
        }
    }
}

Official Bubble Sort Documentation


24. Explain the concept of the Factory Method design pattern.

Answer:

The Factory Method pattern is a creational design pattern that provides an interface for creating instances of a class, with its subclasses deciding which class to instantiate. It allows a class to delegate the responsibility of instantiating objects to its subclasses.

Official Factory Method Pattern Documentation


25. What is the purpose of the State design pattern?

Answer:

The State pattern allows an object to change its behavior when its internal state changes. This pattern is useful in scenarios where an object needs to switch between different states and perform different actions based on its current state.

Official State Pattern Documentation


26. Write a function to perform matrix multiplication.

Answer:

public int[][] multiplyMatrices(int[][] A, int[][] B) {
    int m = A.length, n = A[0].length, p = B[0].length;
    int[][] result = new int[m][p];

    for(int i = 0; i < m; i++) {
        for(int j = 0; j < p; j++) {
            for(int k = 0; k < n; k++) {
                result[i][j] += A[i][k] * B[k][j];
            }
        }
    }

    return result;
}

Official Matrix Multiplication Documentation


27. Explain the concept of the Visitor design pattern.

Answer:

The Visitor pattern is a behavioral design pattern that lets you define a new operation without changing the classes of the elements on which it operates. It’s useful when you have a set of related classes and want to perform operations that are not part of their common interface.

Official Visitor Pattern Documentation


28. What is the purpose of the Command design pattern?

Answer:

The Command pattern is a behavioral design pattern that turns a request into a stand-alone object, encapsulating all information about the request. This allows for parameterization, queuing, and support for operations like undo and redo.

Official Command Pattern Documentation


29. Write a function to check if a binary tree is balanced.

Answer:

public boolean isBalanced(TreeNode root) {
    if (root == null) return true;
    return Math.abs(height(root.left) - height(root.right)) <= 1 &&
           isBalanced(root.left) &&
           isBalanced(root.right);
}

private int height(TreeNode node) {
    if (node == null) return -1;
    return 1 + Math.max(height(node.left), height(node.right));
}

Official Binary Tree Documentation


30. Explain the concept of the Proxy design pattern.

Answer:

The Proxy pattern is a structural design pattern that allows an object (proxy) to act as a substitute for another object. It’s used to control access to the original object or add additional behaviors, like caching, logging, or security checks.

Official Proxy Pattern Documentation


31. What is the purpose of the Chain of Responsibility design pattern?

Answer:

The Chain of Responsibility pattern is a behavioral design pattern that passes a request along a chain of handlers. Each handler decides either to process the request or pass it to the next handler. It decouples senders and receivers of requests.

Official Chain of Responsibility Pattern Documentation


32. Write a function to find the intersection of two arrays.

Answer:

public int[] intersection(int[] nums1, int[] nums2) {
    Set<Integer> set1 = new HashSet<>();
    for (int num : nums1) set1.add(num);
    Set<Integer> set2 = new HashSet<>();
    for (int num : nums2) set2.add(num);

    set1.retainAll(set2);
    int[] result = new int[set1.size()];
    int i = 0;
    for (int num : set1) result[i++] = num;

    return result;
}

Official Intersection of Arrays Documentation


33. Explain the concept of the Interpreter design pattern.

Answer:

The Interpreter pattern is a behavioral design pattern that is used to evaluate sentences in a language. It involves defining a grammar and an interpreter that interprets sentences in the language.

Official Interpreter Pattern Documentation


34. What is the purpose of the Memento design pattern?

Answer:

The Memento pattern is a behavioral design pattern that allows an object’s internal state to be captured and restored at a later time. It’s useful for implementing undo functionalities.

Official Memento Pattern Documentation


35. Write a function to perform a depth-first search (DFS) on a graph.

Answer:

public void dfs(Graph graph, Vertex start) {
    Set<Vertex> visited = new HashSet<>();
    dfsHelper(graph, start, visited);
}

private void dfsHelper(Graph graph, Vertex vertex, Set<Vertex> visited) {
    visited.add(vertex);
    System.out.print(vertex + " ");

    for (Vertex neighbor : graph.getNeighbors(vertex)) {
        if (!visited.contains(neighbor)) {
            dfsHelper(graph, neighbor, visited);
        }
    }
}

Official Depth-First Search Documentation


36. Explain the concept of the Mediator design pattern.

Answer:

The Mediator pattern is a behavioral design pattern that defines an object (mediator) that centralizes communication between objects in a system. It promotes loose coupling and reduces dependencies between objects.

Official Mediator Pattern Documentation


37. What is the purpose of the Null Object design pattern?

Answer:

The Null Object pattern is a behavioral design pattern that uses polymorphism to provide a default behavior for objects. It’s useful in scenarios where an absence of a valid object needs to be handled gracefully, without causing errors.

Official Null Object Pattern Documentation


38. Write a function to calculate the factorial of a number.

Answer:

public int factorial(int n) {
    if (n == 0) return 1;
    return n * factorial(n - 1);
}

Official Factorial Documentation


39. Explain the concept of the Strategy design pattern.

Answer:

The Strategy pattern is a behavioral design pattern that defines a family of interchangeable algorithms and allows them to be used interchangeably within a context object. It enables selecting an algorithm’s implementation at runtime.

Official Strategy Pattern Documentation


40. What is the purpose of the Template Method design pattern?

Answer:

The Template Method pattern is a behavioral design pattern that defines the skeleton of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

Official Template Method Pattern Documentation


41. Write a function to check if a string is an anagram of another string.

Answer:

public boolean isAnagram(String s, String t) {
    if (s.length() != t.length()) return false;

    int[] charCount = new int[26];
    for (int i = 0; i < s.length(); i++) {
        charCount[s.charAt(i) - 'a']++;
        charCount[t.charAt(i) - 'a']--;
    }

    for (int count : charCount) {
        if (count != 0) return false;
    }

    return true;
}

Official Anagram Documentation


42. Explain the concept of the Builder design pattern.

Answer:

The Builder pattern is a creational design pattern that separates the construction of a complex object from its representation, allowing the same construction process to create different representations. It’s particularly useful when an object has a large number of parameters.

Official Builder Pattern Documentation


43. What is the purpose of the Decorator design pattern?

Answer:

The Decorator pattern is a structural design pattern that allows behavior to be added to individual objects, either statically or dynamically, without affecting the behavior of other objects from the same class. It’s used to extend functionalities in a flexible way.

Official Decorator Pattern Documentation


44. Write a function to perform a topological sort on a directed graph.

Answer:

public List<Vertex> topologicalSort(Graph graph) {
    Stack<Vertex> stack = new Stack<>();
    Set<Vertex> visited = new HashSet<>();

    for (Vertex vertex : graph.getVertices()) {
        if (!visited.contains(vertex)) {
            topologicalSortHelper(vertex, visited, stack);
        }
    }

    List<Vertex> result = new ArrayList<>();
    while (!stack.isEmpty()) {
        result.add(stack.pop());
    }
    return result;
}

private void topologicalSortHelper(Vertex vertex, Set<Vertex> visited, Stack<Vertex> stack) {
    visited.add(vertex);
    for (Vertex neighbor : vertex.getNeighbors()) {
        if (!visited.contains(neighbor)) {
            topologicalSortHelper(neighbor, visited, stack);
        }
    }
    stack.push(vertex);
}

Official Topological Sort Documentation


45. Explain the concept of the Abstract Factory design pattern.

Answer:

The Abstract Factory pattern is a creational design pattern that provides an interface for creating families of related or dependent objects without specifying their concrete classes. It’s useful in scenarios where the system needs to be independent of how its objects are created.

Official Abstract Factory Pattern Documentation


46. Write a function to check if a linked list has a cycle.

Answer:

public boolean hasCycle(ListNode head) {
    if (head == null || head.next == null) return false;

    ListNode slow = head;
    ListNode fast = head.next;

    while (slow != fast) {
        if (fast == null || fast.next == null) return false;
        slow = slow.next;
        fast = fast.next.next;
    }

    return true;
}

Official Linked List Cycle Documentation


47. Explain the concept of the Prototype design pattern.

Answer:

The Prototype pattern is a creational design pattern that allows creating new objects by copying an existing object, known as the prototype. This pattern is useful when the cost of creating an object is more expensive than copying an existing one.

Official Prototype Pattern Documentation


48. What is the purpose of the Visitor design pattern?

Answer:

The Visitor pattern is a behavioral design pattern that lets you define a new operation without changing the classes of the elements on which it operates. It’s useful when you have a set of related classes and want to perform operations that are not part of their common interface.

Official Visitor Pattern Documentation


49. Write a function to perform a breadth-first search (BFS) on a graph.

Answer:

public void bfs(Graph graph, Vertex start) {
    Set<Vertex> visited = new HashSet<>();
    Queue<Vertex> queue = new LinkedList<>();
    visited.add(start);
    queue.offer(start);

    while (!queue.isEmpty()) {
        Vertex vertex = queue.poll();
        System.out.print(vertex + " ");

        for (Vertex neighbor : graph.getNeighbors(vertex)) {
            if (!visited.contains(neighbor)) {
                visited.add(neighbor);
                queue.offer(neighbor);
            }
        }
    }
}

Official Breadth-First Search Documentation


50. Explain the concept of the Command design pattern.

Answer:

The Command pattern is a behavioral design pattern that turns a request into a stand-alone object, encapsulating all information about the request. This allows for parameterization, queuing, and support for operations like undo and redo.

Official Command Pattern Documentation


51. What is the purpose of the Chain of Responsibility design pattern?

Answer:

The Chain of Responsibility pattern is a behavioral design pattern that passes a request along a chain of handlers. Each handler decides either to process the request or pass it to the next handler. It decouples senders and receivers of requests.

Official Chain of Responsibility Pattern Documentation


52. Write a function to reverse a linked list.

Answer:

public ListNode reverseLinkedList(ListNode head) {
    ListNode prev = null;
    ListNode current = head;

    while (current != null) {
        ListNode next = current.next;
        current.next = prev;
        prev = current;
        current = next;
    }

    return prev;
}

Official Reverse Linked List Documentation


53. Explain the concept of the Observer design pattern.

Answer:

The Observer pattern is used when one object (the subject) maintains a list of its dependents (observers) and notifies them of state changes, usually by calling one of their methods. This pattern is widely used for implementing distributed event handling systems.

Official Observer Pattern Documentation


54. What is the purpose of the Flyweight design pattern?

Answer:

The Flyweight pattern is used to minimize memory usage and/or computational expenses by sharing as much as possible with related objects. It’s particularly useful when a large number of similar objects need to be created.

Official Flyweight Pattern Documentation


55. Write a function to find the shortest path in a weighted graph using Dijkstra’s algorithm.

Answer:

public void dijkstra(Graph graph, Vertex source) {
    PriorityQueue<Vertex> pq = new PriorityQueue<>(Comparator.comparingInt(v -> v.distance));
    source.distance = 0;
    pq.add(source);

    while (!pq.isEmpty()) {
        Vertex u = pq.poll();

        for (Edge e : u.adjacents) {
            Vertex v = e.target;
            int weight = e.weight;

            if (u.distance + weight < v.distance) {
                v.distance = u.distance + weight;
                v.previous = u;
                pq.add(v);
            }
        }
    }
}

Official Dijkstra’s Algorithm Documentation


56. Explain the concept of Design by Contract.

Answer:

Design by Contract is a software development approach where the interactions between software components are defined by preconditions, postconditions, and invariants. It helps establish a clear understanding of the expected behavior of each component.

Official Design by Contract Documentation


57. Write a function to perform a binary search on a sorted array.

Answer:

public int binarySearch(int[] arr, int target) {
    int left = 0, right = arr.length - 1;

    while (left <= right) {
        int mid = left + (right - left) / 2;

        if (arr[mid] == target) {
            return mid;
        }

        if (arr[mid] < target) {
            left = mid + 1;
        } else {
            right = mid - 1;
        }
    }

    return -1;
}

Official Binary Search Documentation


58. Explain the concept of Memoization in dynamic programming.

Answer:

Memoization is a technique used to optimize recursive algorithms by storing the results of expensive function calls and returning the cached result when the same inputs occur again. It’s commonly used in dynamic programming to avoid redundant calculations and significantly improve performance.

Official Memoization Documentation


59. What is the purpose of the State design pattern?

Answer:

The State pattern allows an object to change its behavior when its internal state changes. This pattern is useful in scenarios where an object needs to switch between different states and perform different actions based on its current state. It helps maintain clean and organized state transitions.

Official State Pattern Documentation


60. Write a function to implement a bubble sort algorithm.

Answer:

public void bubbleSort(int[] arr) {
    int n = arr.length;
    for (int i = 0; i < n-1; i++) {
        for (int j = 0; j < n-i-1; j++) {
            if (arr[j] > arr[j+1]) {
                int temp = arr[j];
                arr[j] = arr[j+1];
                arr[j+1] = temp;
            }
        }
    }
}

Official Bubble Sort Documentation


61. Explain the concept of the Factory Method design pattern.

Answer:

The Factory Method pattern is a creational design pattern that provides an interface for creating instances of a class, with its subclasses deciding which class to instantiate. It allows a class to delegate the responsibility of instantiating objects to its subclasses, providing flexibility in object creation.

Official Factory Method Pattern Documentation


62. What is the purpose of the Decorator design pattern?

Answer:

The Decorator pattern is a structural design pattern that allows behavior to be added to individual objects, either statically or dynamically, without affecting the behavior of other objects from the same class. It’s used to extend functionalities in a flexible way, enabling the composition of behaviors.

Official Decorator Pattern Documentation


63. Write a function to perform matrix multiplication.

Answer:

public int[][] multiplyMatrices(int[][] A, int[][] B) {
    int m = A.length, n = A[0].length, p = B[0].length;
    int[][] result = new int[m][p];

    for(int i = 0; i < m; i++) {
        for(int j = 0; j < p; j++) {
            for(int k = 0; k < n; k++) {
                result[i][j] += A[i][k] * B[k][j];
            }
        }
    }

    return result;
}

Official Matrix Multiplication Documentation


64. Explain the concept of the Singleton design pattern.

Answer:

The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance. It’s particularly useful when one object needs to coordinate actions across the system, such as a logger or a database connection.

Official Singleton Pattern Documentation


65. Write a function to check if a string is a palindrome.

Answer:

public boolean isPalindrome(String s) {
    s = s.replaceAll("[^a-zA-Z0-9]", "").toLowerCase();
    int left = 0, right = s.length() - 1;

    while (left < right) {
        if (s.charAt(left) != s.charAt(right)) {
            return false;
        }
        left++;
        right--;
    }

    return true;
}

Official Palindrome Documentation


66. Explain the concept of the Adapter design pattern.

Answer:

The Adapter pattern is a structural design pattern that allows the interface of an existing class to be used as another interface. It’s often used when a class needs to work with others that have incompatible interfaces. The adapter acts as a wrapper that translates one interface to another.

Official Adapter Pattern Documentation


67. What is the purpose of the Command design pattern?

Answer:

The Command pattern is a behavioral design pattern that turns a request into a stand-alone object, encapsulating all information about the request. This allows for parameterization, queuing, and support for operations like undo and redo.

Official Command Pattern Documentation


68. Write a function to find the GCD (Greatest Common Divisor) of two numbers.

Answer:

public int findGCD(int a, int b) {
    while (b != 0) {
        int temp = b;
        b = a % b;
        a = temp;
    }
    return Math.abs(a);
}

Official GCD Documentation


69. Explain the concept of the Bridge design pattern.

Answer:

The Bridge pattern is a structural design pattern that separates the abstraction from its implementation, allowing them to vary independently. It’s useful when you want to avoid a permanent binding between an abstraction and its implementation.

Official Bridge Pattern Documentation


70. Write a function to calculate the Fibonacci sequence.

Answer:

public int fibonacci(int n) {
    if (n <= 1) return n;
    int[] fib = new int[n + 1];
    fib[0] = 0;
    fib[1] = 1;

    for (int i = 2; i <= n; i++) {
        fib[i] = fib[i - 1] + fib[i - 2];
    }

    return fib[n];
}

Official Fibonacci Sequence Documentation


71. Explain the concept of the Proxy design pattern.

Answer:

The Proxy pattern is a structural design pattern that provides a surrogate or placeholder for another object to control access to it. It can be used for various purposes such as lazy loading, access control, monitoring, and logging, without changing the original object’s code.

Official Proxy Pattern Documentation


72. What is the purpose of the Interpreter design pattern?

Answer:

The Interpreter pattern is a behavioral design pattern that defines a grammar for interpreting a language and provides an interpreter to evaluate sentences in that language. It’s useful when you need to interpret and execute a language or expression.

Official Interpreter Pattern Documentation


73. Write a function to implement a depth-first search (DFS) on a tree.

Answer:

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) {
        val = x;
    }
}

public void dfs(TreeNode root) {
    if (root == null) return;
    System.out.print(root.val + " ");
    dfs(root.left);
    dfs(root.right);
}

Official Depth-First Search on Tree Documentation


74. Explain the concept of the Command design pattern.

Answer:

The Command pattern is a behavioral design pattern that turns a request into a stand-alone object, encapsulating all information about the request. This allows for parameterization, queuing, and support for operations like undo and redo.

Official Command Pattern Documentation


75. What is the purpose of the Chain of Responsibility design pattern?

Answer:

The Chain of Responsibility pattern is a behavioral design pattern that passes a request along a chain of handlers. Each handler decides either to process the request or pass it to the next handler. It decouples senders and receivers of requests.

Official Chain of Responsibility Pattern Documentation


76. Write a function to find the LCM (Least Common Multiple) of two numbers.

Answer:

public int findLCM(int a, int b) {
    return (a * b) / findGCD(a, b);
}

public int findGCD(int a, int b) {
    while (b != 0) {
        int temp = b;
        b = a % b;
        a = temp;
    }
    return Math.abs(a);
}

Official LCM Documentation


77. Explain the concept of the Memento design pattern.

Answer:

The Memento pattern is a behavioral design pattern that allows an object’s internal state to be captured and restored at a later time. This is achieved by externalizing an object’s internal state, so it can be restored to that state without exposing its internal structure.

Official Memento Pattern Documentation


78. Write a function to check if a binary tree is balanced.

Answer:

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) {
        val = x;
    }
}

public boolean isBalanced(TreeNode root) {
    if (root == null) return true;

    int leftHeight = getHeight(root.left);
    int rightHeight = getHeight(root.right);

    return Math.abs(leftHeight - rightHeight) <= 1 && isBalanced(root.left) && isBalanced(root.right);
}

private int getHeight(TreeNode node) {
    if (node == null) return 0;
    return 1 + Math.max(getHeight(node.left), getHeight(node.right));
}

Official Balanced Binary Tree Documentation


79. Explain the concept of the Strategy design pattern.

Answer:

The Strategy pattern is a behavioral design pattern that defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows the algorithm to vary independently from the context that uses it, providing flexibility in selecting the appropriate algorithm.

Official Strategy Pattern Documentation


80. What is the purpose of the Mediator design pattern?

Answer:

The Mediator pattern is a behavioral design pattern that defines an object that centralizes communication between objects in a system. It promotes loose coupling by preventing objects from referring to each other explicitly and instead communicating through the mediator.

Official Mediator Pattern Documentation


81. Write a function to perform a binary search on a binary search tree (BST).

Answer:

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) {
        val = x;
    }
}

public TreeNode searchBST(TreeNode root, int val) {
    if (root == null || root.val == val) return root;

    if (val < root.val) {
        return searchBST(root.left, val);
    } else {
        return searchBST(root.right, val);
    }
}

Official Binary Search in BST Documentation


82. Explain the concept of the Iterator design pattern.

Answer:

The Iterator pattern is a behavioral design pattern that provides a way to access elements of a collection sequentially, without exposing the underlying representation. It decouples the client code from the internal structure of the collection, allowing for flexible iteration.

Official Iterator Pattern Documentation


83. Write a function to perform a topological sort on a directed graph.

Answer:

class Graph {
    private int V;
    private LinkedList<Integer>[] adjList;

    public Graph(int v) {
        V = v;
        adjList = new LinkedList[v];
        for (int i = 0; i < v; ++i) {
            adjList[i] = new LinkedList<>();
        }
    }

    public void addEdge(int v, int w) {
        adjList[v].add(w);
    }

    public void topologicalSort() {
        Stack<Integer> stack = new Stack<>();
        boolean[] visited = new boolean[V];

        for (int i = 0; i < V; i++) {
            if (!visited[i]) {
                topologicalSortUtil(i, visited, stack);
            }
        }

        while (!stack.isEmpty()) {
            System.out.print(stack.pop() + " ");
        }
    }

    private void topologicalSortUtil(int v, boolean[] visited, Stack<Integer> stack) {
        visited[v] = true;

        for (Integer neighbor : adjList[v]) {
            if (!visited[neighbor]) {
                topologicalSortUtil(neighbor, visited, stack);
            }
        }

        stack.push(v);
    }
}

Official Topological Sort Documentation


84. Explain the concept of the Observer design pattern.

Answer:

The Observer pattern is used when one object (the subject) maintains a list of its dependents (observers) and notifies them of state changes, usually by calling one of their methods. This pattern is widely used for implementing distributed event handling systems.

Official Observer Pattern Documentation


85. What is the purpose of the State design pattern?

Answer:

The State pattern allows an object to change its behavior when its internal state changes. This is achieved by representing different states as objects and delegating the behavior to the current state object. It helps maintain clean and organized state transitions.

Official State Pattern Documentation


86. Write a function to find the shortest path in a weighted graph using Bellman-Ford algorithm.

Answer:

class Edge {
    int src, dest, weight;
    Edge() {
        src = dest = weight = 0;
    }
}

class Graph {
    int V, E;
    Edge edge[];

    Graph(int v, int e) {
        V = v;
        E = e;
        edge = new Edge[e];
        for (int i=0; i<e; ++i)
            edge[i] = new Edge();
    }

    void BellmanFord(int src) {
        int dist[] = new int[V];
        Arrays.fill(dist, Integer.MAX_VALUE);
        dist[src] = 0;

        for (int i=1; i<V; ++i) {
            for (int j=0; j<E; ++j) {
                int u = edge[j].src;
                int v = edge[j].dest;
                int weight = edge[j].weight;
                if (dist[u]!=Integer.MAX_VALUE &&
                        dist[u]+weight<dist[v])
                    dist[v]=dist[u]+weight;
            }
        }

        for (int j=0; j<E; ++j) {
            int u = edge[j].src;
            int v = edge[j].dest;
            int weight = edge[j].weight;
            if (dist[u] != Integer.MAX_VALUE &&
                    dist[u] + weight < dist[v])
                System.out.println("Graph contains negative weight cycle");
        }
        printArr(dist);
    }

    void printArr(int dist[]) {
        System.out.println("Vertex Distance from Source");
        for (int i=0; i<V; ++i)
            System.out.println(i + "\t\t" + dist[i]);
    }
}

Official Bellman-Ford Algorithm Documentation


87. Explain the concept of the Builder design pattern.

Answer:

The Builder pattern is a creational design pattern that separates the construction of a complex object from its representation. It allows the same construction process to create different representations of the object. It’s useful when an object needs to be created with a large number of optional parameters.

Official Builder Pattern Documentation


88. What is the purpose of the Flyweight design pattern?

Answer:

The Flyweight pattern is used to minimize memory usage and/or computational expenses by sharing as much as possible with related objects. It’s particularly useful when a large number of similar objects need to be created.

Official Flyweight Pattern Documentation


89. Write a function to implement a priority queue.

Answer:

import java.util.PriorityQueue;

public class CustomPriorityQueue {
    public static void main(String[] args) {
        PriorityQueue<Integer> pq = new PriorityQueue<>();
        pq.add(10);
        pq.add(20);
        pq.add(15);
        System.out.println("Priority Queue: " + pq);
        System.out.println("Top Element: " + pq.peek());
        pq.poll();
        System.out.println("After removing top element: " + pq);
    }
}

Official Priority Queue Documentation


90. Explain the concept of the Chain of Responsibility design pattern.

Answer:

The Chain of Responsibility pattern is a behavioral design pattern that passes a request along a chain of handlers. Each handler decides either to process the request or pass it to the next handler. It decouples senders and receivers of requests.

Official Chain of Responsibility Pattern Documentation


91. What is the purpose of the Null Object design pattern?

Answer:

The Null Object pattern is a behavioral design pattern that uses polymorphism to provide a default behavior for an object. It’s particularly useful in scenarios where you want to avoid null checks and ensure that an object always has a valid reference.

Official Null Object Pattern Documentation


92. Write a function to reverse a linked list.

Answer:

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
    }
}

public ListNode reverseLinkedList(ListNode head) {
    ListNode prev = null;
    ListNode current = head;
    ListNode nextNode = null;

    while (current != null) {
        nextNode = current.next;
        current.next = prev;
        prev = current;
        current = nextNode;
    }

    return prev;
}

Official Reverse Linked List Documentation


93. Explain the concept of the Template Method design pattern.

Answer:

The Template Method pattern is a behavioral design pattern that defines the skeleton of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure. It provides a template for how an algorithm should be executed, allowing customization by subclasses.

Official Template Method Pattern Documentation


94. What is the purpose of the Visitor design pattern?

Answer:

The Visitor pattern is a behavioral design pattern that allows adding further operations to objects without having to modify them. It’s particularly useful when you have a hierarchy of elements and want to perform various operations on them.

Official Visitor Pattern Documentation


95. Write a function to implement a binary search algorithm.

Answer:

public int binarySearch(int[] arr, int target) {
    int left = 0, right = arr.length - 1;

    while (left <= right) {
        int mid = left + (right - left) / 2;

        if (arr[mid] == target)
            return mid;

        if (arr[mid] < target)
            left = mid + 1;
        else
            right = mid - 1;
    }

    return -1;
}

Official Binary Search Documentation


96. Explain the concept of the Abstract Factory design pattern.

Answer:

The Abstract Factory pattern is a creational design pattern that provides an interface for creating families of related or dependent objects without specifying their concrete classes. It allows for the creation of objects with a common theme while keeping the actual types of objects created hidden.

Official Abstract Factory Pattern Documentation


97. Write a function to implement a depth-first search (DFS) on a graph.

Answer:

import java.util.*;

class Graph {
    private int V;
    private LinkedList<Integer>[] adjList;

    Graph(int v) {
        V = v;
        adjList = new LinkedList[v];
        for (int i = 0; i < v; ++i)
            adjList[i] = new LinkedList<>();
    }

    void addEdge(int v, int w) {
        adjList[v].add(w);
    }

    void DFSUtil(int v, boolean[] visited) {
        visited[v] = true;
        System.out.print(v + " ");

        for (Integer neighbor : adjList[v]) {
            if (!visited[neighbor])
                DFSUtil(neighbor, visited);
        }
    }

    void DFS(int v) {
        boolean[] visited = new boolean[V];
        DFSUtil(v, visited);
    }
}

Official Depth-First Search on Graph Documentation


98. Explain the concept of the Prototype design pattern.

Answer:

The Prototype pattern is a creational design pattern that allows creating new objects by copying an existing object, known as the prototype. It’s particularly useful when the cost of creating an object is more expensive than copying an existing one.

Official Prototype Pattern Documentation


99. What is the purpose of the Decorator design pattern?

Answer:

The Decorator pattern is a structural design pattern that allows behavior to be added to individual objects, either statically or dynamically, without affecting the behavior of other objects from the same class. It’s used to extend or alter the functionality of objects at runtime.

Official Decorator Pattern Documentation


100. Write a function to implement a bubble sort algorithm.

Answer:

public void bubbleSort(int[] arr) {
    int n = arr.length;
    for (int i = 0; i < n-1; i++) {
        for (int j = 0; j < n-i-1; j++) {
            if (arr[j] > arr[j+1]) {
                int temp = arr[j];
                arr[j] = arr[j+1];
                arr[j+1] = temp;
            }
        }
    }
}

Official Bubble Sort Documentation