130. Surrounded Regions

题意： 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 XX O O XX X O XX O X X

After running your function, the board should be:

X X X XX X X XX X X XX O X X

思路：这题如果从被包围的O向外操作，这个过程是很繁琐的，而且很难code，所以只要从边界的O入手就可以了，和边界的O相连的O就是不会被翻转的O，所以这题用dfs和bfs都是可以的，把与边界相连的O都标记出来就好了，之后遍历的时候将他们反转就好了。

这段是别人的参考代码（DFS）：

class Solution {public:    void solve(vector<vector<char>>& board) {            int i,j;            int row=board.size();            if(!row)                return;            int col=board[0].size();        for(i=0;i<row;i++){            check(board,i,0,row,col);            if(col>1)                check(board,i,col-1,row,col);        }        for(j=1;j+1<col;j++){            check(board,0,j,row,col);            if(row>1)                check(board,row-1,j,row,col);        }        for(i=0;i<row;i++)            for(j=0;j<col;j++)                if(board[i][j]=='O')                    board[i][j]='X';        for(i=0;i<row;i++)            for(j=0;j<col;j++)                if(board[i][j]=='1')                    board[i][j]='O';    }    void check(vector<vector<char> >&vec,int i,int j,int row,int col){        if(vec[i][j]=='O'){            vec[i][j]='1';            if(i>1)                check(vec,i-1,j,row,col);            if(j>1)                check(vec,i,j-1,row,col);            if(i+1<row)                check(vec,i+1,j,row,col);            if(j+1<col)                check(vec,i,j+1,row,col);        }    }};

下面还有一段是python的代码，因为写的太巧了，特别记录下来：

def solve(self, board):    if not any(board): return    m, n = len(board), len(board[0])    save = [ij for k in range(max(m,n)) for ij in ((0, k), (m-1, k), (k, 0), (k, n-1))]    while save:        i, j = save.pop()        if 0 <= i < m and 0 <= j < n and board[i][j] == 'O':            board[i][j] = 'S'            save += (i, j-1), (i, j+1), (i-1, j), (i+1, j)    for row in board:        for i, c in enumerate(row):            row[i] = 'XO'[c == 'S']

save中存储所有可能的边界情况。