The #1 skill that decides your placement. Learn every core data structure with plain-English explanations, then prove it by solving real interview problems in our built-in online judge — no signup, everything free.
⚡ Start Solving Problems →Each topic below maps directly to problems in the practice judge — learn the idea, then immediately solve.
The foundation of everything. Contiguous memory, O(1) access by index. Interviews love array manipulation: traversals, in-place edits, prefix sums, and the two-pointer pattern.
Strings are arrays of characters with extra tricks: frequency counting, palindromes, anagrams, and sliding windows. Service-company coding rounds are ~40% string problems.
Key → value lookup in O(1). The single most useful interview tool: turns O(n²) brute-force into O(n). In JavaScript: Map, Set, or plain objects.
Stack = Last-In-First-Out (undo, brackets, recursion). Queue = First-In-First-Out (scheduling, BFS). The "valid parentheses" problem is asked in nearly every fresher interview.
Nodes connected by pointers — O(1) insert/delete, O(n) access. Interviewers use linked lists to test whether you can reason about references. "Reverse a linked list" is the classic.
Linear search checks everything: O(n). Binary search halves the space each step: O(log n) — but only on sorted data. Off-by-one bugs here fail more interviews than any other topic.
Bubble/Selection/Insertion are O(n²) teaching tools; Merge Sort and Quick Sort are the O(n log n) real deals. Interviews rarely ask you to write a full sort — they ask about the merge step and complexity trade-offs.
The pattern behind most "substring/subarray" problems. Maintain a moving window and shrink/grow it instead of re-checking everything — O(n²) becomes O(n). Zoho and Freshworks love these.
Break a problem into overlapping subproblems and remember answers. Start with Fibonacci and Climbing Stairs, then Coin Change. Needed for TCS Digital/Prime and product companies — not for Ninja-level rounds.
The one table to memorize before any technical interview.
| Structure / Algorithm | Access | Search | Insert | Delete | Notes |
|---|---|---|---|---|---|
| Array | O(1) | O(n) | O(n) | O(n) | Insert/delete shift elements |
| HashMap | — | O(1)* | O(1)* | O(1)* | *average case |
| Linked List | O(n) | O(n) | O(1) | O(1) | O(1) if you have the node |
| Stack / Queue | O(n) | O(n) | O(1) | O(1) | Only at the ends |
| Binary Search | — | O(log n) | — | — | Sorted data only |
| Bubble / Insertion Sort | — | — | O(n²) | Fine for tiny inputs | |
| Merge / Quick Sort | — | — | O(n log n) | Merge = stable; Quick = in-place | |
| BST (balanced) | O(log n) | O(log n) | O(log n) | O(log n) | Degrades to O(n) if skewed |
2 hours a day. By week 6 you're ready for any service-company round and most product-company screens.
Learn traversal, reversal, frequency counting. Solve problems 1–10 in the practice judge (all Easy). Aim for 2 per day.
Re-solve Two Sum and Anagram using maps in O(n). Then Move Zeroes and Remove Duplicates with two pointers.
Valid Parentheses, then Reverse Linked List. Draw the pointer diagrams on paper — never solve linked lists in your head.
Binary Search until the off-by-ones stop biting. Implement the merge step. Understand why Quick Sort is O(n log n) average, O(n²) worst.
Longest Substring, Max Subarray (Kadane), Climbing Stairs, Fibonacci iteratively. These four unlock half of all Medium problems.
Coin Change, Product Except Self, Trapping Rain Water. Then do timed mocks: 2 problems in 45 minutes, explaining your approach out loud. Pair with our TCS NQT guide and interview question bank.
DSA pairs with everything else on this site: