题解 | #牛牛学数列6#

#include <iostream>
using namespace std;


int main(){
    int n;
    int a=0,b=1,c=1,d;
    cin>>n;
    if(n==1){
        cout<<0;
    }
    else if(n==2){
        cout<<1;
    }
    else if(n==3){
        cout<<1;
    }
    else{
        for(int m=3;m<n;m++){
            d = a + 2*b + c;
            a = b;
            b = c;
            c = d;
        
        }
    }
    cout<<d;
    
    
    
    
    return 0;
}