LeetCode 200 ·
Medium
Number of Islands
Count islands (4-connected groups of land) in a grid of 1s and 0s.
Try it
Step through the core mechanic. The simulator below runs the dfs shape this problem is built on.
Interactive · grid flood-fill
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stack (bottom → top)
(0,0)
seed the stack with the start cell (0,0)
BFS vs DFS, same grid.
Swap the frontier from a queue to a stack and breadth-first becomes depth-first.
Both visit exactly the same cells; only the order differs. BFS layers outward
(shortest-path / wave problems); DFS plunges down one branch (component / path problems).
The approach
Scan the grid; on each unvisited land cell, flood-fill (DFS or BFS) the whole island to mark it visited, then increment the count. Each cell is touched once.
| Aspect | Value |
|---|---|
| Pattern | DFS |
| Recognise it by | Count connected components on a grid. |
| Time complexity | O(mn) |
| Space complexity | O(mn) |
| Difficulty | Medium |
Who asks it
Companies known to ask this problem, from public LeetCode company-tag aggregations. A signal of where to expect it, not a guarantee.