LeetCode

Interviewers always ask 'what's the complexity?' — you must answer confidently and compare options.

The most common interview category — and a HashMap turns O(n²) solutions into O(n).

A simple pattern that turns nested O(n²) search into a single O(n) pass — great on sorted arrays.

Any 'longest/shortest contiguous subarray' question is usually a sliding-window problem.

The stack elegantly solves balanced-parentheses and next-greater-element problems.

Beyond sorted arrays — binary search on the answer space cracks surprisingly hard problems.

Linked-list problems test your pointer control — a skill interviewers love to see.

Tree problems are among the most common categories — most solve with clean recursion.

Heaps crack top-K problems fast, and interval problems show up often in scheduling.

Islands, course schedules, and networks are all graphs — mastering BFS/DFS covers a huge category.

When the ask is 'all combinations' — permutations, subsets, combination sum — backtracking is the tool.

The hardest interview category — but once you recognize the patterns, they repeat predictably.

Solving alone is one thing; solving while explaining under time pressure is another — practice readies you for the real thing.