C :

#include<stdio.h>
#include<string.h>
int main()
{
    int n,sum;
    while(1)
    {
        char a[20000];
        scanf("%s",a);
        if(a[0]=='0')break;
        sum=0;
        n=strlen(a);
        for(int i=0;i<n;i++)         //首次处理,期望回十亿之内 
        {
            sum+=(a[i]-'0');          //计数 
            //printf("%d\t",sum);
        }
        sum%=9; 
        if(sum==0)printf("9\n");
        else printf("%d\n",sum);
    }/**/
    return 0;
}

C++ :

#include <stdio.h>

int DigitalRoot(int num){
	int root = num;
	while(root >= 10){
		root = 0;
		while(num){
			root += num%10;
			num /= 10;
		}
		num = root;
	}
	return root;
}

int main(){
	char strN[1000];
	while(scanf("%s",strN), strN[0]!='0'){
		int i=0, n=0;
		while(strN[i]){
			n += strN[i]-'0';
			i++;
		}
		printf("%d\n", DigitalRoot(n));
	}

	return 0;
}