题解

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

const int maxn = 1e7+10;
const int mod = 1e9+7;
long long a[maxn];

int main() {

	a[1] = 1;
	a[2] = 2;
	a[3] = 4; 
	for(int i = 4; i <= 1e7; i++) {
		a[i] = (a[i-1]%mod + a[i-2]%mod + a[i-3]%mod)%mod; // 求余公式
	}

	int t;
	cin >> t;
	cout << a[t] << endl; 
	return 0;
}