Number of Islands

Given an m × n 2D binary grid where '1' represents land and '0' represents water, return the number of islands.

An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically.

Example 1:

Input:
grid = [
  ["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:
grid = [
  ["1","1","0","0","0"],
  ["1","1","0","0","0"],
  ["0","0","1","0","0"],
  ["0","0","0","1","1"]
]
Output: 3

Constraints: