用递推

#include <bits/stdc++.h>
using namespace std;
int main(){
    int n,a[40];
    a[1]=1;
    a[2]=2;
    a[3]=4;
    cin >> n;
    for(int i = 4;i <= n;i++){
        a[i]=a[i-1]+a[i-2]+a[i-3];
    }
    cout << a[n];
    return 0;
}
//本少爷专属水印[:色眯眯][:色眯眯][:色眯眯]