130. 被围绕的区域.md

class Solution {
    public void solve(char[][] board) {
        boolean[][] zt = new boolean[board.length][board[0].length];
        int len1 = board.length - 1, len2 = board[0].length - 1;
        for (int i = 0; i <= len1; i++) {
            solveDFS(board, zt, i, 0);
            solveDFS(board, zt, i, len2);
        }
        for (int i = 0; i <= len2; i++) {
            solveDFS(board, zt, 0, i);
            solveDFS(board, zt, len1, i);
        }
        for (int i = 0; i < len1; i++)
            for (int j = 0; j < len2; j++)
                if (board[i][j] == 'O' && !zt[i][j]) board[i][j] = 'X';
    }

    private void solveDFS(char[][] board, boolean[][] zt, int i, int j) {
        if (zt[i][j] || board[i][j] == 'X') return;
        zt[i][j] = true;
        if (i > 0) solveDFS(board, zt, i - 1, j);
        if (j > 0) solveDFS(board, zt, i, j - 1);
        if (i < board.length - 1) solveDFS(board, zt, i + 1, j);
        if (j < board[0].length - 1) solveDFS(board, zt, i, j + 1);
    }
}