Leetcode: Surrounded Regions @Python

class Solution:
    # @param board, a 9x9 2D array
    # Capture all regions by modifying the input board in-place.
    # Do not return any value.
    def solve(self, board):
        def fill(x, y):
            if x<0 or x>m-1 or y<0 or y>n-1 or board[x][y] != 'O': return
            queue.append((x,y))
            board[x][y]='D'
        def bfs(x, y):
            if board[x][y]=='O':queue.append((x,y)); fill(x,y)
            while queue:
                curr=queue.pop(0); i=curr[0]; j=curr[1]
                fill(i+1,j);fill(i-1,j);fill(i,j+1);fill(i,j-1)
        if len(board)==0: return
        m=len(board); n=len(board[0]); queue=[]
        for i in range(n):
            bfs(0,i); bfs(m-1,i)
        for j in range(1, m-1):
            bfs(j,0); bfs(j,n-1)
        for i in range(m):
            for j in range(n):
                if board[i][j] == 'D': board[i][j] = 'O'
                elif board[i][j] == 'O': board[i][j] = 'X'