[Leetcode][JAVA] Surrounded Regions

Given a 2D board containing 'X' and '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


基本思路是：如果把所有不该变为X的O标注出来，那么剩下的O就是要变为X的了。
所以，从矩阵各个边上的O开始进行图的遍历，入栈/队列时把相应节点值从‘O'变成其他符号（我用的是'+'）,出栈/队列时对节点四方的相邻值为'O'的节点进行讨论。
全部遍历完后，把'O'变为'X'， '+'变为'O'。
代码：

public class Pair {
        int x;
        int y;
        Pair(int x, int y) {
            this.x = x;
            this.y = y;
        }
    }
    public void solve(char[][] board) {
        int m=board.length;
        if(m<=2)
            return;
        int n=board[0].length;
        for(int i=0;i<m;i++) {
            if(board[i][0]=='O')
                dfs(board, i, 0);
            if(board[i][n-1]=='O')
                dfs(board, i, n-1);
        }
        for(int i=0;i<n;i++) {
            if(board[0][i]=='O')
                dfs(board, 0, i);
            if(board[m-1][i]=='O')
                dfs(board, m-1, i);
        }
        for(int i=0;i<m;i++)
            for(int j=0;j<n;j++) {
                if(board[i][j]=='O')
                    board[i][j]='X';
                else if(board[i][j]=='+')
                    board[i][j]='O';
            }
    }
    public void dfs(char[][] bd, int i, int j)
    {
        Stack<Pair> st = new Stack<Pair>();
        bd[i][j] = '+';
        st.push(new Pair(i,j));
        
        while(!st.isEmpty()) {
            Pair temp = st.pop();
            if(temp.x>0 && bd[temp.x-1][temp.y]=='O') {
                bd[temp.x-1][temp.y]='+';
                st.push(new Pair(temp.x-1, temp.y));
            }
            if(temp.y<bd[0].length-1 && bd[temp.x][temp.y+1]=='O') {
                bd[temp.x][temp.y+1]='+';
                st.push(new Pair(temp.x, temp.y+1));
            }
            if(temp.x<bd.length-1 && bd[temp.x+1][temp.y]=='O') {
                bd[temp.x+1][temp.y]='+';
                st.push(new Pair(temp.x+1, temp.y));
            }
            if(temp.y>0 && bd[temp.x][temp.y-1]=='O') {
                bd[temp.x][temp.y-1]='+';
                st.push(new Pair(temp.x, temp.y-1));
            }
        }
    }