Leetcode 417. Pacific Atlantic Water Flow

Given an m×n height grid where water can flow from a cell to any neighbor of lesser-or-equal height, return all coordinates from which water can reach both the Pacific (top/left edges) and Atlantic (bottom/right edges). The core challenge is a graph/grid reachability problem typically solved by two multi-source traversals (BFS/DFS) from the ocean borders, propagating uphill (to equal-or-higher heights) and intersecting the reachable sets.