20 hard LeetCode picks
Ojas Sharma posted a curated list of 20 hard LeetCode problems aimed at Google interview prep — includes interval and tree variants like 'Employees Free Time' and 'Number of Islands in a Tree'. The list is positioned for depth over memorization and targets high‑value algorithm patterns. (x.com)
Ojas Sharma’s public LeetCode profile lists 273 problems solved in C++ and a contest rating of 1,551 on the platform’s leaderboard. (leetcode.com) “Employee Free Time” is LeetCode problem 759 and standard solutions flatten all employees’ interval lists, sort by start time, then scan for gaps to produce the free-time intervals. (walkccc.me) Complexity analysis for that approach is commonly reported as O(mn log(mn)) time and O(mn) space where m is number of employees and n is average intervals per employee. (leetcode.doocs.org) “Number of Islands” corresponds to LeetCode problem 200 and the official LeetCode listing shows 4,047,213 accepted submissions with a 64.0% acceptance rate as of the problem page. (leetcode.com) LeetCode maintains a curated “Top Google Questions” problem list that many interview-prep curators cross-reference when assembling Google-focused hard-problem sets. (leetcode.com) Community-maintained Google prep collections are long-standing: the GitHub repo “google-interview-preparation-problems” by mgechev has drawn thousands of stars (≈3.2k) as a collective reference, and recent community posts have compiled “hard” LeetCode picks for OA seasons (e.g., a Codeforces blog compiling must-solve hard problems in 2025). (github.com) (codeforces.com) The two example problems named in the thread map cleanly to canonical patterns — 759 tests interval-merge/line-sweep skills while 200 tests grid/graph traversal (DFS/BFS/Union-Find) — and both problem pages list those topic tags on their LeetCode entries. (walkccc.me) (leetcode.com) Original post and full 20-problem thread are hosted on X at the provided status URL. (x.com)
