动态规划方法

    1. int numWays(int n){
    2.     if (n == 0) return 1;
    3.     if (n == 1) return 1;
    4.     if (n == 2) return 2;
    6.     unsigned long long t1 = 2, t2 = 1, result = 0;
    8.     for (int i = 3;i <= n;++i) {
    9.         result = (t1 + t2)%1000000007;
    10.         t2 = t1;
    11.         t1 = result;
    12.     }
    14.     if (result == 1000000008) return 1;
    16.     return result;
    17. }