ZOJ 1115 Digital Roots

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

using namespace std;

int main()
{
string num;

while (cin >> num, num[0] != '0')
{
while (num.length() > 1)
{
string sum = "0";
for (int i = 0; i < num.length(); i++)
{
int temp = num[i] - '0';
int index = 0;

while (temp > 0)
{
if (index == sum.length())
{
sum += "0";
}

int s = (sum[index] - '0') + temp;
sum[index++] = '0' + s % 10;
temp = s / 10;
}
}

num = sum;
}

printf("%s\n", num.c_str());
}

return  0;
}