4 articles
Find the maximum area of any island in a binary grid using DFS that returns the size of each connected component. The canonical "DFS with return value" pattern asked at Google, Meta, and Amazon.
LC 1905 Count Sub Islands asks how many islands in grid2 are subsets of islands in grid1. The critical trap is short-circuiting DFS on the first invalid cell — you must always complete the traversal to mark the whole island visited, while tracking validity separately.
Flip at most one 0 to 1 to maximize island area. The "color the islands, then try every flip" trick avoids quadratic re-DFS — a top Google interview problem.
Count connected components by Union Find (decrement count on each successful union) or by BFS / DFS (increment count for each unvisited node). Both run in near-linear time.