130. 被围绕的区域

https://leetcode-cn.com/problems/surrounded-regions/comments/
两次遍历。第一次先遍历边界，dfs将所有与边界O相连的O置为‘-’。之后再遍历所有，将所有O变为X，所有’-‘变为O

class Solution:
    def solve(self, board):
        """
        :type board: List[List[str]]
        :rtype: void Do not return anything, modify board in-place instead.
        """
        n = len(board)
        if n == 0:
            return
        m = len(board[0])
        #对四条边界上的元素dfs遍历
        for i in range(n):            
            self.dfs(i, 0, n, m, board)
            self.dfs(i, m-1, n, m, board)
        for j in range(m):
            self.dfs(0, j, n, m, board)
            self.dfs(n-1, j, n, m, board)
        #遍历所有
        for i in range(n):
            for j in range(m):
            	#注意先后顺序不能颠倒
                if board[i][j] == 'O':
                    board[i][j] = 'X'
                if board[i][j] == '-':
                    board[i][j] = 'O'
        
    def dfs(self, row, col, rows, cols, board):
        if row >= 0 and row < rows and col >= 0 and col < cols and board[row][col] == 'O':
            board[row][col] = '-'  #特殊标记
            #dfs遍历上下左右
            self.dfs(row, col-1, rows, cols, board)
            self.dfs(row, col+1, rows, cols, board)
            self.dfs(row-1, col, rows, cols, board)
            self.dfs(row+1, col, rows, cols, board)