机器人的运动范围

public class Solution {
    int threshold;
    public int movingCount(int threshold, int rows, int cols) {
        // DFS
        this.threshold = threshold;
        int[][] flag = new int[rows][cols];
        return helper(0, 0, rows, cols, flag);
    }


    private int helper(int i, int j, int row, int col, int[][] flag) {
        if(i < 0 || i >= row || j < 0 || j >= col || flag[i][j] == 1 
           || numSum(i) + numSum(j) > threshold) {
            return 0;
        }
        flag[i][j] = 1; // 标记已经走过
        // 只需要先向下走再一层一层向右走，回溯
        return helper(i + 1, j, row, col, flag)
                + helper(i, j + 1, row, col, flag) + 1;
    }

    private int numSum(int num) {
        int sum = 0;
        do {
            sum += num % 10;
        } while((num = num / 10) > 0);
        return sum;
    }
}