斐波那契数列高级版

#include<bits/stdc++.h>
using namespace std;
long long a[10000005];
int main(){
    int n;
    cin >> n;
    a[1] = 1;
    a[2] = 2;
    a[3] = 4;
    for(int i = 4;i <= n;i ++){
        a[i] = (a[i - 1] + a[i - 2] + a[i - 3]) % 1000000007;
    }
    cout << a[n];
    return 0;
}