Heap

LeetCode 23 note: a heap that holds only k heads, and divide-and-conquer pairwise merging

heapq basics, Top-K, lazy deletion, two heaps, k-way merge, sweep line, and Dijkstra

LeetCode 253 note: a heap of end times for running meetings; half-open intervals need <=

LeetCode 716 note: store unique ids on both the stack and the heap, record deletions by id, and discard deleted entries lazily; tracking values cannot tell duplicates apart

LeetCode 2973 note: the max product of three is either the three largest or the two smallest times the largest, so each subtree returns only its 2 smallest and 3 largest values, merged in post-order

LeetCode 4046 note: Dijkstra with the last direction and the turns used in the state; for the same cell and direction, an earlier pop is cheaper, and if it also used fewer turns it dominates later states

LeetCode 1425 note: dp[i] = nums[i] + max(0, max of the previous k dp values); that max is a sliding window maximum, available in O(1) from a monotonic deque

LeetCode 1354 note: work backwards, since the maximum was set last; jump with a modulo when the gap is large

LeetCode 499 note: Dijkstra prioritized by (distance, path string), and the ball stops when it rolls over the hole

LeetCode 2163 note: try every boundary k between the two parts; the left takes the n smallest of nums[:k] (a max-heap) and the right the n largest of nums[k:] (a min-heap), one prefix and one suffix pass

LeetCode 2102 note: two heaps for “top k” and “the rest”, with a custom lt for reversed string order

LeetCode 1878 note: enumerate every top vertex and size, walk the four edges, and keep the three largest distinct sums