DSA Interview Questions and Answers for Freshers

Two-sum and its variants (use a hash map, O(n), state it early). Find duplicates (hash set or sort). Majority element (hash count or Boyer-Moore). Maximum subarray sum (Kadane, O(n)). Subarray sum to target (prefix sum map).

Interview presentation: clarify constraints (sorted? negatives? size), state brute force and complexity, then the optimal solution, then edge cases. That exact order scores even when the code is simple.

Anagram checks (frequency array or sort), longest substring without repeating characters (sliding window), reverse words in a sentence, and string-to-int style parsing problems.

String problems test edge-case discipline: empty strings, single characters, unicode or spaces. Mention them out loud before writing code.

Reverse a linked list (iterative and recursive), detect a cycle (two pointers), merge two sorted lists, valid parentheses (stack), and queue-using-stacks style problems.

These are favorite warm-up questions — they are quick and reveal whether fundamentals are real.

Binary tree traversals (iterative and recursive), maximum depth, lowest common ancestor, and graph problems like connected components and shortest path in a grid (BFS).

Practice BFS with a visited array under time limits — grid problems with variations (obstacles, multiple sources) appear in OAs of product companies.

Climbing stairs, house robber, unique paths, and coin-change style problems. Recognize the state, write the memo, and verify on small examples.

If a problem asks for count, max, or min with overlapping subproblems, DP is the expected route at medium level. State the recurrence before coding.