59. 岛屿数量.md

给你一个由 '1'（陆地）和 '0'（水）组成的的二维网格，请你计算网格中岛屿的数量。岛屿总是被水包围，并且每座岛屿只能由水平方向和/或竖直方向上相邻的陆地连接形成。此外，你可以假设该网格的四条边均被水包围。

输入：grid = [
  ["1","1","0","0","0"],
  ["1","1","0","0","0"],
  ["0","0","1","0","0"],
  ["0","0","0","1","1"]
]
输出：3

class Solution:
    def numIslands(self, grid: List[List[str]]) -> int:
        count = 0
        #构造递归
        def dfs(i, j):
            #递归终止条件
            if not 0<=i<len(grid) or not 0<=j<len(grid[0]) or grid[i][j] == '0':
                return
            grid[i][j] = '0'
            #情况1
            dfs(i+1,j)
            #情况2
            dfs(i-1,j)
            #情况3
            dfs(i,j+1)
            #情况4
            dfs(i,j-1)

        for i in range(len(grid)):
            for j in range(len(grid[0])):
                if grid[i][j] == '1':
                    #调用递归
                    dfs(i,j)
                    count += 1
        #返回要优化的目标
        return count