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Grid as an Implicit Graph
A 2D matrix where each cell is a node and neighbors are edges — no adjacency list ever built. The most common disguise a graph wears in interviews.
What to know
- Number of Islands, Rotting Oranges, Word Search are all graph problems on grids.
- Directions array [(0,1),(1,0),(0,-1),(-1,0)] replaces adjacency lists.
- Multi-source BFS starts the queue with every source cell at distance 0.
- Visited state can be stored in the grid itself to save memory.
In the wild: Image editing flood fill, game maps, maze solving, wildfire/epidemic spread simulation.
Algorithms to reach for
Flood fill (DFS/BFS)
O(rows · cols)Count islands, fill regions
Multi-source BFS
O(rows · cols)Nearest-exit / rotting-oranges style spreading
0-1 BFS (deque)
O(rows · cols)Shortest path when edges cost 0 or 1
Practice problems — with full guides
Each problem has its own page: progressive hints, how to approach it, and the full solution.