力扣： 剑指 Offer 10- II. 青蛙跳台阶问题

class Solution {
    public int numWays(int n) {
        int a = 1, b = 1, c = 1;
        int mod = 1000000007;
        while (n > 1){
            c = (a + b) % mod;
            a = b;
            b = c;
            n -- ;
        }
        return c;
    }
}

其实就是斐波那契数