唯一的坑點就是，輸入的數字太長，超過 long long 的範圍。所以用字串接受，先來一遍，然後轉化成數字即可。

題目傳送門

#include <algorithm>
#include  <iostream>
#include   <cstring>
#include   <stdio.h>
#include    <string>
#include    <vector>
#include     <stack>
#include     <cmath> 
#include     <deque>
#include   	 <queue>
#include       <map>

using namespace std;
typedef long long ll;
const int MOD = 1e9+7;

#define Foru(i,a,b) for(i = a;i < b;i ++)
#define Ford(i,a,b) for(i = a;i > b; i --) 
#define MAXN 100001

int main(){

	while(true){
		char ch;
		ll sum = 0;
		while(true){
			ch = getchar();
			if(ch == '\n'){
				break;
			}
			sum += ch - '0';
		}
		if(sum == 0)break;
		ll stem;
		while(sum >= 10){
			stem = sum;
			sum = 0;
			while(stem != 0){
				sum += stem % 10;
				stem /= 10;
			}
		}
		printf("%d\n",sum);
	} 
	
	return 0;
}