注意遍歷的順序，從四周開始。與Pacific Atlantic Water Flow類似。

class Solution(object):
    def solve(self, board):
        """
        :type board: List[List[str]]
        :rtype: void Do not return anything, modify board in-place instead.
        """
        visit=set()
        change=set()
        m=len(board)
 

        if m==0: return 
        n=len(board[0])
        def dfs(board,x,y,visit):
            visit.add((x,y))
            res={(x,y)}
            for dx,dy in zip((-1,0,1,0),(0,-1,0,1)):
                nx,ny=x+dx,y+dy
                if nx<0 or nx>=len(board) or ny<0 or ny>=len(board[0]) or board[
 
nx][ny]=='X' or (nx,ny) in visit:
                    continue
                res=res| dfs(board,nx,ny,visit)
            return res 
        for i in range(m):
            if board[i][0] == 'O' and (i,0) not in visit:
                change=change|dfs(board,i,0,visit)
        for i in range(m):
            if
 
 board[i][n-1] == 'O' and (i,n-1) not in visit:
                change=change|dfs(board,i,n-1,visit)
        for j in range(1,n-1):
            if board[0][j]=='O' and (0,j) not in visit:
                change=change|dfs(board,0,j,visit)
        for j in range(1,n-1):
            if board[m-1][j]=='O' and (m-1,j) not in visit:
                change=change|dfs(board,m-1,j,visit)
        for i in range(m):
            for j in range(n):
                if board[i][j]=='X' or (i,j) in change:
                    continue 
                board[i][j]='X'