surrounded_regions.cpp

//思路：
/*
    First, check the four border of the matrix. If there is a element is
    'O', alter it and all its neighbor 'O' elements to '1'.
    Then ,alter all the 'O' to 'X'
    At last,alter all the '1' to 'O'
 */
class Solution {
public:
    void solve(vector<vector<char>>& board) {
        int row = board.size();
        if(row==0)
            return;
        int col = board[0].size();
        for(int i=0;i<row;i++){
            check(board,i,0,row,col);
            if(col>1)
                check(board,i,col-1,row,col);
        }
        for(int i=1;i<col-1;i++){
            check(board,0,i,row,col);
            if(row>1)
                check(board,row-1,i,row,col);
        }
        for(int i=0;i<row;i++)
            for(int j=0;j<col;j++){
                if(board[i][j]=='1')
                    board[i][j]='O';
                else if(board[i][j]=='O')
                    board[i][j]='X';
            }
        }
    void check(vector<vector<char>>& board,int i,int j,int row,int col){
        if(board[i][j]=='O'){
            board[i][j]='1';
            if(i>1)
                check(board,i-1,j,row,col);
            if(i+1<row)
                check(board,i+1,j,row,col);
            if(j>1)
                check(board,i,j-1,row,col);
            if(j+1<col)
                check(board,i,j+1,row,col);
        }
    }
};