#477 Surrounded Regions-优快云博客

本文介绍了一种解决二维棋盘中被X包围的O区域捕获问题的有效算法。通过首先标记边缘O及其相连区域，再将剩余O设为X，最后将标记的.设回O，实现了对特定区域的精准捕获。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

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.

Have you met this question in a real interview?

Yes

Example

X X X X
X O O X
X X O X
X O X X

After capture all regions surrounded by 'X', the board should be:

X X X X
X X X X
X X X X
X O X X

题目思路：

这题可以反向思维：如果需要把所有被‘X’包围的‘O’改为‘X’，那么唯一可能不被‘X’包围的情况就是‘O’在board的边上，或者‘O’和在board边上的‘O’相连。这样题目就转化为找在board边上的‘O’以及它们的同党。这样，就先找到所有在board边上的‘O’，对于每一个这样的‘O’，上下左右找它们的同党，并把这类的‘O’都标记为‘.’以作区别。

找完同党以后，就可以把所有剩下的‘O’，也就是那些被‘X’包围的‘O’设为‘X’，而后把那些‘.’设为‘O’了。

Mycode（AC = 26ms）：

class Solution {
public:
    /**
     * @param board a 2D board containing 'X' and 'O'
     * @return void
     */
    void surroundedRegions(vector<vector<char>>& board) {
        // Write your code here
        int row = board.size();
        if (row == 0) return;
        
        int col = board[0].size();
        
        // for each 'O' on the edges, do bfs
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                if (i == 0 || j == 0 || i == row - 1 || j == col - 1) {
                    surroundedRegionsHelper(board, i, j);
                }
            }
        }
        
        // set 'O' to 'X', and set '.' to 'O'
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                if (board[i][j] == 'O') {
                    board[i][j] = 'X';
                }
            }
        }
        
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                if (board[i][j] == '.') {
                    board[i][j] = 'O';
                }
            }
        }
    }
    
    // return each edge linked 'O' into '.'
    void surroundedRegionsHelper(vector<vector<char>>& board, int row, int col) {
        if (board[row][col] == 'X' || board[row][col] == '.') {
            return;
        }
        
        int dx[4] = {1, 0, -1, 0};
        int dy[4] = {0, 1, 0, -1};
        
        board[row][col] = '.';
        for (int i = 0; i < 4; i++) {
            int x = row + dx[i];
            int y = col + dy[i];
            
            if (x >= 0 && y >= 0 && x < board.size() && y < board[0].size()) {
                surroundedRegionsHelper(board, x, y);
            }
        }
        
        return;
    }
};