Description: Given an m x n 2D binary grid which represents a map of '1's (land) and '0's (water), return the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Optimal Approach

Use BFS or DFS to find the number of connected components, or starting nodes for BFS or DFS.

Time: O(nm)

Space: O(nm) for Queue (BFS) or Recursive stack space (DFS)