【LeetCode 130. 被围绕的区域】

1. 题目

2. 分析

这题其实非常不错。如果正向解，非常麻烦；因为很难界定哪些O是被包围的？但是如果反向解呢？因为边界的O不会被包围，那么就可以想到跟边界O相连的O都不会被包围。那么除此之外的O都会被包围，题目就解决了。

3. 代码

class Solution:
    def solve(self, board: List[List[str]]) -> None:
        """
        Do not return anything, modify board in-place instead.
        """
        m, n = len(board), len(board[0])
        vis = [[0] * n for i in range(m)]
        
        # 只从边界遍历
        for i in [0,m-1]:
            for j in range(n):
                if board[i][j] == 'O':                
                    self.dfs(i, j, m, n, vis, board)
        for j in [0, n-1]:
            for i in range(m):
                if board[i][j] == 'O':                
                    self.dfs(i, j, m, n, vis, board)

        print(vis)
        for i in range(m):
            for j in range(n):
                if vis[i][j] == 0:
                    board[i][j] = 'X'
            
    def dfs(self, i, j, m, n, vis, board):
        if i>=0 and j>=0 and i<m and j< n:
            if vis[i][j] == 0 and board[i][j] == "O":
                vis[i][j] = 1
                for item in [(i-1,j), (i, j-1), (i+1, j), (i, j+1)]:
                    new_i, new_j = item
                    self.dfs(new_i, new_j, m, n, vis, board)