Number of Islands

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 surrounded by water.

Examples

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

Explanation: There is one island formed by all the connected '1's in the top-left region.

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

Explanation: There are three islands: top-left, center, and bottom-right.

Example 3:

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

Output: 5

Explanation: Each '1' is isolated, so there are 5 separate islands.

Constraints

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