面试题 08.01. 三步问题

面试题 08.01. 三步问题

简单的dp

class Solution {
public:
    int waysToStep(int n) {
        if(n <= 2)
            return n;
        long a = 1, b = 1, c = 2;
        for(int i = 3; i <= n; ++i)
        {
            a = (c + b + a) % 1000000007;
            swap(a, c);
            swap(a, b);
        }
        return c;
    }
};