gold-8-1

三步问题。有个小孩正在上楼梯，楼梯有n阶台阶，小孩一次可以上1阶、2阶或3阶。实现一种方法，计算小孩有多少种上楼梯的方式。结果可能很大，你需要对结果模1000000007。

示例1:

输入：n = 3 
输出：4
说明: 有四种走法

示例2:

输入：n = 5
输出：13

提示:

1. n范围在[1, 1000000]之间

//leetcode submit region begin(Prohibit modification and deletion)
class Solution {
    public int waysToStep(int n) {
        if (n == 1) {
            return 1;
        }
        if (n == 2) {
            return 2;
        }
        if (n == 3) {
            return 4;
        }

        int a1 = 1;
        int a2 = 2;
        int a3 = 4;

        int cur = 0;

        for (int i = 4; i <= n; i++) {
            cur = ((a1 + a2) % 1000000007 + a3) % 1000000007;
            a1 = a2;
            a2 = a3;
            a3 = cur;
        }

        return cur;
    }
}
//leetcode submit region end(Prohibit modification and deletion)