HDU 1013不解释

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

char s[1001];
int Roots(int x)
{
    int num=0,i;
    while( x )
    {
       num += x%10;
       x /= 10;
    }   
    if(num < 10) 
       return num;
    else
       return Roots(num); 
}    
    
int main()
{
    int sum,i, ans;
        
    while(scanf("%s",s) != EOF)
    {
        if(s[0] == '0') break;                 
         sum = 0;                   
        for(i=0; i<strlen(s); i++)
          sum += (s[i]-'0');
         if(sum < 10) 
         {
            printf("%d\n",sum);
         }
         else
         {
            ans = Roots(sum);
            printf("%d\n",ans);
         }     
       
   }
   return 0;
}