题解 | #牛群的活动区域# Java

把与边界相连的B的块块置为C

最后全遍历一遍

• 是B变为C
• 是C变为B

public class Solution {

    public char[][] solve (char[][] board) {
        int m = board.length;
        int n = board[0].length;
        for (int i = 0; i < m; i++) {
            dfs(board, i, 0);
            dfs(board, i, n - 1);
        }
        for (int j = 0; j < n; j++) {
            dfs(board, 0, j);
            dfs(board, m - 1, j);
        }

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (board[i][j] == 'B') board[i][j] = 'A';
                else if (board[i][j] == 'C') board[i][j] = 'B';
            }
        }
        return board;
    }

    void dfs(char[][] board, int i, int j) {
        int m = board.length;
        int n = board[0].length;
        if (i < 0 || i >= m || j < 0 || j >= n || board[i][j] != 'B') {
            return;
        }
        board[i][j] = 'C';
        dfs(board, i - 1, j);
        dfs(board, i, j - 1);
        dfs(board, i + 1, j);
        dfs(board, i, j + 1);
    }
}