【重点】【DFS】1254.统计封闭岛屿的数目

题目

DFS

Python

思路：先外后内，先遍历边界，再处理内部！！！

class Solution:
    def closedIsland(self, grid: List[List[int]]) -> int:
        res = 0
        m, n = len(grid), len(grid[0])

        for i in range(m): # 先遍历边界0, 并置1
            for j in range(n):
                if (i == 0 or i == m-1 or j == 0 or j == n-1) and grid[i][j] == 0:
                    self.dfs(grid, i, j)

        for i in range(m): # 再遍历内部0, 都是封闭岛
            for j in range(n):
                if grid[i][j] == 0:
                    self.dfs(grid, i, j)
                    res += 1
        return res
    
    def dfs(self, grid, i, j):
        if (i < 0 or i >= len(grid) 
            or j < 0 or j >= len(grid[0])
            or grid[i][j] == 1):
            return
        grid[i][j] = 1
        self.dfs(grid, i-1, j)
        self.dfs(grid, i+1, j)
        self.dfs(grid, i, j+1)
        self.dfs(grid, i, j-1)