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. 
 1 class Solution(object):
 2     def solve(self, board):
 3         """
 4         :type board: List[List[str]]
 5         :rtype: void Do not return anything, modify board in-place instead.
 6         """
 7         
 8         if not board:
 9             return
10         
11         m = len(board)
12         n = len(board[0])
13         
14         queue = []
15         
16         for i in range(m):
17             for j in range(n):
18                 if i == 0 or j == 0 or i == m -1 or j == n-1:
19                     if board[i][j] == 'O':
20                         queue.append([i,j])
21                     
22         while queue:
23             [p,q] = queue.pop(0)
24             board[p][q] ='V'
25             if self.isValid(p+1, q, m, n) and board[p+1][q] == 'O':
26                 queue.append([p+1, q])
27             if self.isValid(p-1, q, m, n) and board[p-1][q] == 'O':
28                 queue.append([p-1, q])
29             if self.isValid(p, q+1, m, n) and board[p][q+1] == 'O':
30                 queue.append([p, q+1])
31             if self.isValid(p, q-1, m, n) and board[p][q-1] == 'O':
32                 queue.append([p, q-1])
33 
34         for i in range(m):
35             for j in range(n):
36                 if board[i][j] == 'V':
37                     board[i][j] ='O'
38                 elif board[i][j] == 'O':
39                     board[i][j] ='X'
40     
41     def isValid(self, i,j, m, n):
42         return i>0 and i<m and j>0 and j<n
43     
44

 
posted @ 2016-11-18 14:35  lettuan  阅读(151)  评论(0)    收藏  举报