Given an `m x n`

2D binary grid `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: 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:**

`m == grid.length`

`n == grid[i].length`

`1 <= m, n <= 300`

`grid[i][j]`

is`'0'`

or`'1'`

.

Solution:

Time complexity: O(N²)

Space complexity: O(N)