题解

#include<iostream>
#include<map>
#include<string>
using namespace std;
const long long q=1e9+7;
long long a[10000005];
int main(){
int b;
cin>>b;
a[1]=1;
a[2]=2;
a[3]=4;
for(int i=4;i<=b;i++){
a[i]=(a[i-1]+a[i-2]+a[i-3])%q;
}
cout<<a[b];
}