130. 被围绕的区域(dfs)

//由于与边界相连的O都不会被填充，可以反向考虑从边界出发寻找连通域，
//将这些O设置成Y。剩下在区域中的O就都会被填充
class Solution {
    private int row;
    private int col;
    public void solve(char[][] board) {
        if (board == null || board.length == 0 || board[0].length == 0) {
            return;
        }
        row = board.length;
        col = board[0].length;
        //行边界搜索
        for (int i = 0; i < col; i++) {
            if (board[0][i] == 'O') {
                dfs(board, 0, i);
            }
            if (board[row - 1][i] == 'O') {
                dfs(board, row - 1, i);
            }
        }
        //列边界搜索
        for (int i = 0; i < row; i++) {
            if (board[i][0] == 'O') {
                dfs(board, i, 0);
            }
            if (board[i][col - 1] == 'O') {
                dfs(board, i, col - 1);
            }
        }

        //将所有Y修改成O,所有O修改成X
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                if (board[i][j] == 'O') {
                    board[i][j] = 'X';
                }
                if (board[i][j] == 'Y') {
                    board[i][j] = 'O';
                }

            }
        }

    }

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