(dfs) $O(n^2)$
C++ 代码
#include<bits/stdc++.h>
#define int long long
using namespace std;
int res;
signed dfs(int n) {
int a=n/3,b=n%3;
res+=a;
if(a==0&&b==0){
return res;
}
if(a+b<3){
return res;
}
dfs(a+b);
}
signed main(){
int n;
scanf("%d",&n);
printf("%d",dfs(n)+n);
}