Divide-and-Conquer

LeetCode 324 note: sort, split into a small half and a large half, reverse each, and interleave with small values at even indices and large ones at odd indices

LeetCode 148 note: split in half with fast and slow pointers, sort each half recursively and merge; O(n log n) with no extra array

LeetCode 240 note: start at the top-right corner, moving left when larger and down when smaller, eliminating a row or column each step

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

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

LeetCode 105 note: the first pre-order value is the root, and its position in the in-order tells the left subtree’s size; look positions up in a dictionary for O(n)

LeetCode 347 note: count first, then take the k most frequent with a size-k heap or a bucket sort indexed by count

LeetCode 169 note: Boyer-Moore voting, where different values cancel in pairs and the survivor is the one appearing more than half the time

LeetCode 108 note: the middle element becomes the root and each half recurses; the halves differ by at most one, so the tree is balanced