参考思路 https://blog.csdn.net/songchuwang1868/article/details/86535150

#include<bits/stdc++.h>
using namespace std;
int arr[22];
int main(){
    arr[0]=1;//arr[0]=3^0;
    for(int i=1;i<=19;i++){
        int t=1;int x=i; 
        while(x--){
            t*=3;
        }
        arr[i]=arr[i-1]+t;
    }//算出arr[1]~arr[19]，arr[1]=3^0+3^1,arr[2]=3^0+3^1+3^2,...
    int n;
    cin>>n;
    if(n==1){
        cout<<1;
        return 0;
    }
    for(int i=0;i<19;i++){
        if(arr[i]<n&&n<=arr[i+1]){
            cout<<i+2<<endl;
            return 0;
        }
    }

}