Leetcode 1970. Last Day Where You Can Still Cross

Given an r×c grid where cells become water each day in a given order, find the last day when there still exists a 4-directional path of land connecting the top row to the bottom row. The core challenge is testing reachability under incremental deletions — typically handled by binary searching the day and checking connectivity (BFS/DFS or union-find) on the resulting land cells.