题解（太简单了

#include<bits/stdc++.h>
using namespace std;

const int a = 1e9 + 7;
long long dp[11451400]; //别问我为什么这么臭（（（
int main(){
    int n;
    cin >> n;
    dp[1] = 1;
    dp[2] = 2;
    dp[3] = 4;
    for (int i = 4;i <= n;i++){
        dp[i] = (dp[i - 1] + dp[i - 2] + dp[i - 3]) % a;
    }
    cout << dp[n];
    return 0;
}