分析

c++

#include<iostream>
#include<cstring>
#include<string>
using namespace std;

string str;

int toSum(string s){
    int n=s.size();
    int sum=0;
    for(int i=0;i<n;i++){
        sum+=s[i]-'0';
    }
    return sum;
}


int main(){
    cin>>str;

    int s=toSum(str);
    while(s>9){
        s=toSum(to_string(s));
    }
    cout<<s<<endl;
    return 0;
}