Array

LeetCode 189 note: take k %= n, then reverse three times: the whole array, the first k, and the last n - k, with O(1) extra space

LeetCode 162 note: nums[mid] < nums[mid + 1] guarantees a peak to the right (rising until the edge at worst), otherwise one at or left of mid

LeetCode 134 note: if total gas is short there is no answer; otherwise scan, and whenever the tank goes negative move the start to the next station

LeetCode 137 note: with everything else appearing three times, count the 1s at each bit modulo 3; the leftover bits form the answer, minding negatives

LeetCode 106 note: the last post-order value is the root; taking roots from the end of the post-order, build the right subtree before the left

LeetCode 1646 note: generate by the rules, nums[i // 2] at even i and nums[i // 2] + nums[i // 2 + 1] at odd i, handling n = 0 separately

LeetCode 48 note: rotating 90 degrees clockwise is a transpose followed by reversing each row, both in place

LeetCode 695 note: DFS each unvisited 1 to measure its island and keep the maximum; cells are the integer 1, not a string

LeetCode 417 note: walk inward from each ocean’s shore to cells no lower than the current one; cells reached from both oceans are the answer

LeetCode 973 note: as in Top-K, keep a size-k max-heap (negated distances) holding the k closest points

LeetCode 287 note: treat i -> nums[i] as a linked list; the duplicate is the cycle entry, found with Floyd’s algorithm in O(1) space without modifying the array

LeetCode 215 note: keep a min-heap of size k whose top is the k-th largest, or use quickselect for O(n) on average