Number of Islands
Count the islands (groups of horizontally or vertically connected land cells) in a grid.
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Lesson notes
Try it before you watch
Restate the problem in your own words, list the edge cases, and sketch a solution with its running time. Then play the video and compare.
Reveal the key idea
Scan the grid; each unvisited land cell starts a new island, and a DFS or BFS flood fill marks the whole island as visited.
Pattern: Matrices & grids. Keep row and column boundaries explicit and treat grid cells as graph nodes when needed.
Complexity
| Measure | Bound |
|---|---|
| Time | O(m · n) |
| Extra space | O(m · n) in the worst case |
Walkthroughs often start from a simpler approach first; aim to reach these bounds. New to Big-O? Read understanding algorithmic complexity.