https://leetcode-cn.com/problems/three-steps-problem-lcci/
class Solution {
public:
const int mod=1e9+7;
int waysToStep(int n)
{
long long int f[1000001]={
0};
f[0]=1,f[1]=1,f[2]=2;
for(int i=3;i<=n;i++) f[i]=(f[i-1]+f[i-2]+f[i-3])%mod;
return f[n];
}
};