Top 25 DSA Interview Questions And Answers (With Code Examples)

🧩 1. What is an Array?

An array is a linear data structure that stores elements in contiguous memory locations.

✅ Common Interview Use: Searching, sorting, sliding window problems.

🔁 2. How to Reverse an Array in Python?

pythonCopyEditarr = [1, 2, 3, 4, 5]
reversed_arr = arr[::-1]
print(reversed_arr)

📚 3. What is a Linked List?

A linked list is a linear data structure where each element is a node pointing to the next one.

✅ Used in: Dynamic memory allocation, stacks, and queues.

🧵 4. Reverse a Linked List

pythonCopyEditdef reverse_list(head):
    prev = None
    while head:
        next = head.next
        head.next = prev
        prev = head
        head = next
    return prev

🌲 5. What is a Binary Tree?

A binary tree is a hierarchical structure where each node has at most two children: left and right.

🔍 6. Inorder Traversal of a Binary Tree

pythonCopyEditdef inorder(root):
    if root:
        inorder(root.left)
        print(root.val)
        inorder(root.right)

🎯 7. What is Binary Search?

A divide-and-conquer algorithm for sorted arrays.

pythonCopyEditdef binary_search(arr, target):
    low, high = 0, len(arr)-1
    while low <= high:
        mid = (low + high)//2
        if arr[mid] == target:
            return mid
        elif arr[mid] < target:
            low = mid + 1
        else:
            high = mid - 1
    return -1

💡 8. What is the Time Complexity of Binary Search?

O(log n)

🧠 9. Stack vs Queue

Stack: LIFO (Last In, First Out)
Queue: FIFO (First In, First Out)

🔄 10. Implement Stack Using List

pythonCopyEditstack = []
stack.append(1)
stack.pop()

📥 11. What is Recursion?

A function calling itself. Used in tree traversal, backtracking, etc.

🧪 12. Fibonacci Using Recursion

pythonCopyEditdef fib(n):
    if n <= 1:
        return n
    return fib(n-1) + fib(n-2)

🧱 13. What is Dynamic Programming?

An optimization technique that solves complex problems by breaking them into overlapping sub-problems.

🪄 14. Memoized Fibonacci

pythonCopyEditdp = {}
def fib(n):
    if n in dp:
        return dp[n]
    if n <= 1:
        return n
    dp[n] = fib(n-1) + fib(n-2)
    return dp[n]

🧮 15. What is a HashMap / Dictionary?

Key-value data structure with O(1) average time for insertion/search.

⚔️ 16. Two Sum Problem (Using Hashmap)

pythonCopyEditdef two_sum(nums, target):
    seen = {}
    for i, num in enumerate(nums):
        diff = target - num
        if diff in seen:
            return [seen[diff], i]
        seen[num] = i

🔄 17. Difference Between BFS and DFS

BFS: Level-order traversal using a queue
DFS: Depth-wise using recursion or stack

🧪 18. Detect Cycle in a Graph

Use DFS with visited + recursion stack or Union-Find.

🏁 19. Topological Sort

Used in dependency resolution (e.g., course scheduling).

🧬 20. What is a Trie?

A tree-like data structure used to store words efficiently.

🧩 21. Sliding Window Technique

Used for subarray problems – keeps track of a subset of data.

📈 22. Kadane’s Algorithm (Max Subarray Sum)

pythonCopyEditdef max_subarray(arr):
    max_sum = current = arr[0]
    for num in arr[1:]:
        current = max(num, current + num)
        max_sum = max(max_sum, current)
    return max_sum

🧠 23. What is a Heap?

A special tree-based structure used in priority queues.

🔁 24. What is Backtracking?

Used to explore all possibilities — like solving Sudoku.

🧪 25. What is Big O Notation?

Describes time/space complexity:

O(1): Constant
O(n): Linear
O(log n): Logarithmic

✅ Conclusion:

These DSA interview questions cover a mix of theory and practical coding problems. Practice them in Python and try solving variations on platforms like:

LeetCode
HackerRank
InterviewBit

Want more deep-dives into specific topics like trees, graphs, or DP? Let us know in the comments or check out our DSA Series ➜

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