Number of Islands — medium DSA Problem

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.

Example 1:

Input:
4 5
1 1 1 1 0
1 1 0 1 0
1 1 0 0 0
0 0 0 0 0
Output: 1

Example 2:

Input:
4 5
1 1 0 0 0
1 1 0 0 0
0 0 1 0 0
0 0 0 1 1
Output: 3

Constraints:

m == grid.length, n == grid[i].length
1 ≤ m, n ≤ 300
grid[i][j] is '0' or '1'

Input format: First line: m n. Then m lines each with n space-separated integers (0 or 1).

Output format: Integer — number of islands.