백준 알고리즘 6378번 : 디지털 루트

링크

https://www.acmicpc.net/problem/6378

sol1)

#pragma GCC target("avx,avx2,fma")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#define fastio ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)
#define int int64_t
using namespace std;

int32_t main(){
  fastio;
   string s;
    int sum = 0;
    while(1){
      cin >> s;
      if(s == "0") break;
      sum = 0;
      while(1){
          for(int i = 0; i < s.length(); i++)
              sum += s[i] - '0';
          
          if(sum < 10)
              break;
          else{
              s = to_string(sum);
              sum = 0;
          }
      }
    cout << sum << '\n';
  }
}