Started grinding LeetCode for interview prep. Day one was grid BFS: Number of Islands, Max Area of Island, Rotting Oranges, and Flood Fill. One skeleton, four twists.
- Mark a cell visited the moment you enqueue it, not when you dequeue it — mark on dequeue and the same cell can pile up in the queue multiple times before it's ever processed
- Counting islands is just: scan every cell, and every time you hit unvisited land, that's a new island — BFS from there so it never gets counted again
- Max Area of Island is the exact same code, it just returns the island's size instead of counting
- Rotting Oranges is multi-source BFS — throw every rotten orange in the queue before the loop starts, then process it one minute (one queue "level") at a time
- Three ways to mark a cell visited: a separate set, overwriting the grid value, or painting a new color in Flood Fill — same idea, different storage
- Flood Fill's one real trap: if the new color is the same as the old one, nothing changes, so nothing ever gets marked — infinite loop unless you return early
- Recursive DFS can go stack-deep on a long snake-shaped island; BFS's queue never gets that deep — worth mentioning out loud in an interview
- Hit three silent bugs today: compared to the string '1' on an integer grid, enqueued the wrong cell, and typed == where I meant = — all three were quiet, no errors, just wrong answers