README.md

200. Number of Islands

Given an m x n 2d grid 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'.

Solutions (Python)

1. DFS

class Solution:
    def numIslands(self, grid: List[List[str]]) -> int:
        m, n = len(grid), len(grid[0])
        stack = []
        ret = 0

        for r0 in range(m):
            for c0 in range(n):
                if grid[r0][c0] == '1':
                    ret += 1
                    grid[r0][c0] = '0'
                    stack.append((r0, c0))
                    while stack:
                        r1, c1 = stack.pop()
                        for (i, j) in [(0, -1), (0, 1), (1, 0), (-1, 0)]:
                            r2, c2 = r1 + i, c1 + j
                            if 0 <= r2 < m and 0 <= c2 < n and grid[r2][c2] == '1':
                                grid[r2][c2] = '0'
                                stack.append((r2, c2))

        return ret