✅ Top 50 DSA (Data Structures & Algorithms) Interview Questions 📚⚙️

1. What is a Data Structure?
2. What are the different types of data structures?
3. What is the difference between Array and Linked List?
4. How does a Stack work?
5. What is a Queue? Difference between Queue and Deque?
6. What is a Priority Queue?
7. What is a Hash Table and how does it work?
8. What is the difference between HashMap and HashSet?
9. What are Trees? Explain Binary Tree.
10. What is a Binary Search Tree (BST)?
11. What is the difference between BFS and DFS?
12. What is a Heap?
13. What is a Trie?
14. What is a Graph?
15. Difference between Directed and Undirected Graph?
16. What is the time complexity of common operations in arrays and linked lists?
17. What is recursion?
18. What are base case and recursive case?
19. What is dynamic programming?
20. Difference between Memoization and Tabulation?
21. What is the Sliding Window technique?
22. Explain Two-Pointer technique.
23. What is the Binary Search algorithm?
24. What is the Merge Sort algorithm?
25. What is the Quick Sort algorithm?
26. Difference between Merge Sort and Quick Sort?
27. What is Insertion Sort and how does it work?
28. What is Selection Sort?
29. What is Bubble Sort and its drawbacks?
30. What is the time and space complexity of sorting algorithms?
31. What is Backtracking?
32. Explain the N-Queens Problem.
33. What is the Kadane's Algorithm?
34. What is Floyd’s Cycle Detection Algorithm?
35. What is the Union-Find (Disjoint Set) algorithm?
36. What are topological sorting and its uses?
37. What is Dijkstra's Algorithm?
38. What is Bellman-Ford Algorithm?
39. What is Kruskal’s Algorithm?
40. What is Prim’s Algorithm?
41. What is Longest Common Subsequence (LCS)?
42. What is Longest Increasing Subsequence (LIS)?
43. What is a Palindrome Substring problem?
44. What is the difference between greedy and dynamic programming?
45. What is Big-O notation?
46. What is the difference between time and space complexity?
47. How to find the time complexity of a recursive function?
48. What are amortized time complexities?
49. What is tail recursion?
50. How do you approach solving a coding problem in interviews?

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✅ Top DSA Interview Questions with Answers: Part-1 🧠

1. What is a Data Structure?
A data structure is a way to organize, store, and manage data efficiently so it can be accessed and modified easily. Examples: Arrays, Linked Lists, Stacks, Queues, Trees, Graphs.

2. What are the different types of data structures?
• Linear: Arrays, Linked Lists, Stacks, Queues
• Non-linear: Trees, Graphs
• Hash-based: Hash Tables, Hash Maps
• Dynamic: Heaps, Tries, Disjoint Sets

3. What is the difference between Array and Linked List?
• Array: Fixed size, index-based access (O(1)), insertion/deletion is expensive
• Linked List: Dynamic size, sequential access (O(n)), efficient insertion/deletion at any position

4. How does a Stack work?
A Stack follows LIFO (Last In, First Out) principle.
• Operations: push() to add, pop() to remove, peek() to view top
• Used in: undo mechanisms, recursion, parsing

5. What is a Queue? Difference between Queue and Deque?
A Queue follows FIFO (First In, First Out).
• Deque (Double-Ended Queue): Allows insertion/removal from both ends.
• Used in scheduling, caching, BFS traversal.

6. What is a Priority Queue?
A type of queue where each element has a priority.
• Higher priority elements are dequeued before lower ones.
• Implemented using heaps.

7. What is a Hash Table and how does it work?
A structure that maps keys to values using a hash function.
• Allows O(1) average-case lookup, insert, delete.
• Handles collisions using chaining or open addressing.

8. What is the difference between HashMap and HashSet?
• HashMap: Stores key-value pairs
• HashSet: Stores only unique keys (no values)
Both use hash tables internally.

9. What are Trees? Explain Binary Tree.
A tree is a non-linear structure with nodes connected hierarchically.
• Binary Tree: Each node has at most 2 children (left, right).
Used in hierarchical data, parsers, expression trees.

10. What is a Binary Search Tree (BST)?
A special binary tree where:
• Left child < Node < Right child
• Enables fast lookup, insert, and delete in O(log n) (average case).
Maintains sorted structure.

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✅ Top DSA Interview Questions with Answers: Part-2 🧠

11. What is the difference between BFS and DFS?
- BFS (Breadth-First Search): Explores neighbors first (level by level). Uses a queue. ➡️
- DFS (Depth-First Search): Explores depth (child nodes) first. Uses a stack or recursion. ⬇️
Used in graph/tree traversals, pathfinding, cycle detection. 🌳🔎

12. What is a Heap?
A binary tree with heap properties:
- Max-Heap: Parent ≥ children 🔼
- Min-Heap: Parent ≤ children 🔽
Used in priority queues, heap sort, scheduling algorithms. ⏰

13. What is a Trie?
A tree-like data structure used to store strings. 🌲
Each node represents a character.
Used in: autocomplete, spell-checkers, prefix search. 🔡

14. What is a Graph?
A graph is a collection of nodes (vertices) and edges. 🔗
- Can be directed/undirected, weighted/unweighted.
Used in: networks, maps, recommendation systems. 🗺️

15. Difference between Directed and Undirected Graph?
- Directed: Edges have direction (A → B ≠ B → A) ➡️
- Undirected: Edges are bidirectional (A — B) ↔️
Used differently based on relationships (e.g., social networks vs. web links).

16. What is the time complexity of common operations in arrays and linked lists?
- Array: 🔢
- Access: O(1)
- Insert/Delete: O(n)
- Linked List: 🔗
- Access: O(n)
- Insert/Delete: O(1) at head

17. What is recursion?
When a function calls itself to solve a smaller subproblem. 🔄
Requires a base case to stop infinite calls.
Used in: tree traversals, backtracking, divide & conquer. 🌳🧩

18. What are base case and recursive case?
- Base Case: Condition that ends recursion 🛑
- Recursive Case: Part where the function calls itself ➡️
Example:

def fact(n):
    if n == 0: return 1   # base case
    return n * fact(n-1)  # recursive case

19. What is dynamic programming?
An optimization technique that solves problems by breaking them into overlapping subproblems and storing their results (memoization). 💾
Used in: Fibonacci, knapsack, LCS. 📈

20. Difference between Memoization and Tabulation?
- Memoization (Top-down): Uses recursion + caching 🧠
- Tabulation (Bottom-up): Uses iteration + table 📊
Both store solutions to avoid redundant calculations.

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✅ Top DSA Interview Questions with Answers: Part-3 🧠

21. What is the Sliding Window technique?
It’s an optimization method used to reduce time complexity in problems involving arrays or strings. You create a "window" over a subset of data and slide it as needed, updating results on the go.
Example use case: Find the maximum sum of any k consecutive elements in an array.

22. Explain the Two-Pointer technique.
This involves using two indices (pointers) to traverse a data structure, usually from opposite ends or the same direction. It's helpful for searching pairs or reversing sequences efficiently.
Common problems: Two-sum, palindrome check, sorted array partitioning.

23. What is the Binary Search algorithm?
It’s an efficient algorithm to find an element in a sorted array by repeatedly dividing the search range in half.
Time Complexity: O(log n)
Key idea: Compare the target with the middle element and eliminate half the array each step.

24. What is the Merge Sort algorithm?
A divide-and-conquer sorting algorithm that splits the array into halves, sorts them recursively, and then merges them.
Time Complexity: O(n log n)
Stable? Yes
Extra space? Yes, due to merging.

25. What is the Quick Sort algorithm?
It chooses a pivot, partitions the array so elements < pivot are left, and > pivot are right, then recursively sorts both sides.
Time Complexity: Avg – O(n log n), Worst – O(n²)
Fast in practice, but not stable.

26. Difference between Merge Sort and Quick Sort
• Merge Sort is stable, consistent in performance (O(n log n)), but uses extra space.
• Quick Sort is faster in practice and works in-place, but may degrade to O(n²) if pivot is poorly chosen.

27. What is Insertion Sort and how does it work?
It builds the sorted list one item at a time by comparing and inserting items into their correct position.
Time Complexity: O(n²)
Best Case (nearly sorted): O(n)
Stable? Yes
Space: O(1)

28. What is Selection Sort?
It finds the smallest element from the unsorted part and swaps it with the beginning.
Time Complexity: O(n²)
Space: O(1)
Stable? No
Rarely used due to inefficiency.

29. What is Bubble Sort and its drawbacks?
It repeatedly compares and swaps adjacent elements if out of order.
Time Complexity: O(n²)
Space: O(1)
Drawback: Extremely slow for large data. Educational, not practical.

30. What is the time and space complexity of common sorting algorithms?
• Bubble Sort → Time: O(n²), Space: O(1), Stable: Yes
• Selection Sort → Time: O(n²), Space: O(1), Stable: No
• Insertion Sort → Time: O(n²), Space: O(1), Stable: Yes
• Merge Sort → Time: O(n log n), Space: O(n), Stable: Yes
• Quick Sort → Avg Time: O(n log n), Worst: O(n²), Space: O(log n), Stable: No

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