啊题解

#include<bits/stdc++.h>
using namespace std;
int main() {
int n;
cin >> n;
long long a[40] = {};
a[1] = 1, a[2] = 2, a[3] = 4;
for(int i = 4; i <= n + 1; i++){
a[i] = a[i - 1] + a[i - 2] + a[i - 3];
}
cout << a[n];
return 0;
}
/*
i | a[i]
1 | 1
2 | 2
3 | 4
4 | 7
*/