Leetcode 130. Surrounded Regions

Given a 2D board containing 'X' and 'O' (the letter O), capture all regions surrounded by 'X'.

A region is captured by flipping all 'O's into 'X's in that surrounded region.

For example,

X X X X
X O O X
X X O X
X O X X

After running your function, the board should be:

X X X X
X X X X
X X X X
X O X X

keys:　1. BFS from the most outer layer. 

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

         if not board:
             return

         m = len(board)
         n = len(board[0])

         queue = []

         for i in range(m):
             for j in range(n):
                 if i == 0 or j == 0 or i == m -1 or j == n-1:
                     if board[i][j] == 'O':
                         queue.append([i,j])

         while queue:
             [p,q] = queue.pop(0)
             board[p][q] ='V'
             if self.isValid(p+1, q, m, n) and board[p+1][q] == 'O':
                 queue.append([p+1, q])
             if self.isValid(p-1, q, m, n) and board[p-1][q] == 'O':
                 queue.append([p-1, q])
             if self.isValid(p, q+1, m, n) and board[p][q+1] == 'O':
                 queue.append([p, q+1])
             if self.isValid(p, q-1, m, n) and board[p][q-1] == 'O':
                 queue.append([p, q-1])

         for i in range(m):
             for j in range(n):
                 if board[i][j] == 'V':
                     board[i][j] ='O'
                 elif board[i][j] == 'O':
                     board[i][j] ='X'

     def isValid(self, i,j, m, n):
         return i>0 and i<m and j>0 and j<n