Top 50 Data Structures & Algorithms (DSA) Interview Questions & Answers 2026

Why Data Structures & Algorithms Matter in Tech Interviews

Data Structures and Algorithms form the foundation of technical screening at top tech companies worldwide. Whether you are interviewing at product-based giants or service-based firms, demonstrating strong problem-solving skills is essential.

1. Array & String Manipulation

Two Sum Problem

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

# Python Hash Map Optimal Solution - O(N) Time, O(N) Space
def twoSum(nums, target):
    seen = {}
    for i, num in enumerate(nums):
        diff = target - num
        if diff in seen:
            return [seen[diff], i]
        seen[num] = i
    return []

2. Linked List Algorithms

Reverse a Singly Linked List

Iterative approach using three pointers (previous, current, next):

// Java Iterative Solution - O(N) Time, O(1) Space
public ListNode reverseList(ListNode head) {
    ListNode prev = null;
    ListNode curr = head;
    while (curr != null) {
        ListNode nextTemp = curr.next;
        curr.next = prev;
        prev = curr;
        curr = nextTemp;
    }
    return prev;
}

3. Tree & Graph Traversal

Understanding Breadth-First Search (BFS) and Depth-First Search (DFS) is mandatory for solving graph and tree problem variations.

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