1582 articles
LeetCode 90 — asked at Amazon, Apple, and Google. Generate all unique subsets from an array with duplicates. Sort first, then in backtracking skip duplicate elements at the same recursion depth. O(2^n) time — the cleanest duplicate-handling pattern in all backtracking problems.
LeetCode 796 — asked at Amazon and Google. Check if string s can become string goal by rotating. The elegant O(n) trick: concatenate s with itself and check if goal is a substring. One line of code once you see the insight.
LeetCode 686 — asked at Google and Amazon. Find the minimum number of times to repeat string a so that b is a substring. The ceiling trick: repeat a at least ceil(len(b)/len(a)) times, then check that and one more. O(n*m) time, O(n+m) space.
LeetCode 42 — one of the most asked hard problems at Amazon, Google, Microsoft, and Meta. Compute trapped rainwater using two pointers in O(n) time and O(1) space. The key: water at any position is min(left_max, right_max) minus the height. Move the pointer with the smaller max inward.
LeetCode 239 — asked at Amazon, Google, and Microsoft. Find the maximum in each sliding window of size k in O(n) using a monotonic decreasing deque. The deque stores indices in decreasing order of value — pop from front when out of window, pop from back when current element is larger.