题目描述：
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
在由'X'和'O'组成的矩阵中，找出所有被'X'包围的'O'，将它们替换为'X'。
思路：
1.本题可以DFS也可以BFS，但是DFS速度会慢一些，无法通过测试数据。无论哪种方式，就是从四个边的'O'作为切入点n（如果BFS，需要先将四个边的'O'入队列），遍历时找到n的上下左右邻居，分别进行标记（假设标记为A），凡是标记为A的点即为：不能被围的'O'，以这些邻居为中心继续下一次标记。
2.然后对board做一个遍历，把剩余的'O'赋值为'X'，而'A'赋值为'O'即可。
实现代码：public class Solution {
public void Solve(char[,] board)
{
var row = board.GetLength(0);
var col = board.GetLength(1);
if(row < 2 || col < 2){
return;
}
// try go from left & right boundary
var q = new Queue<Pos>();
for(var i = 0;i < row; i++){
if(board[i , 0] == 'O'){
q.Enqueue(new Pos(i , 0));
}
if(board[i , col - 1] == 'O'){
q.Enqueue(new Pos(i , col - 1));
}
}
// try go from top & down boundary
for(var i = 0;i < col; i++){
if(board[0,i] == 'O'){
q.Enqueue(new Pos(0 , i));
}
if(board[row - 1 , i] == 'O'){
q.Enqueue(new Pos(row - 1 , i));
}
}
Bfs(ref board, row, col , q);
// restore 'A' to 'O'
// mark 'O' to 'X'
for(var i = 0;i < row; i++){
for(var j = 0;j < col; j++){
if(board[i,j] == 'O'){
board[i,j] = 'X';
}
if(board[i,j] == 'A'){
board[i,j] = 'O';
}
}
}
}
private void Bfs(ref char[,] board, int rowLen, int colLen, Queue<Pos> q)
{
if(q.Count == 0){
return;
}
var q1 = new Queue<Pos>();
while(q.Count > 0){
var p = q.Dequeue();
board[p.row, p.col] = 'A';
// move up
if(p.row > 0 && board[p.row - 1, p.col] == 'O'){
q1.Enqueue(new Pos(p.row - 1, p.col));
}
// move down
if(p.row < rowLen - 1 && board[p.row + 1, p.col] == 'O'){
q1.Enqueue(new Pos(p.row + 1, p.col));
}
// move right
if(p.col < colLen - 1 && board[p.row, p.col + 1] == 'O'){
q1.Enqueue(new Pos(p.row, p.col + 1));
}
// move left
if(p.col > 0 && board[p.row, p.col - 1] == 'O'){
q1.Enqueue(new Pos(p.row, p.col - 1));
}
}
Bfs(ref board, rowLen, colLen, q1);
}
class Pos{
public Pos(int r, int c){
row = r;
col = c;
}
public int row;
public int col;
}
}