HDU-1013-Digital Roots

Digital Roots
Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)
Problem Description
The digital root of a positive integer is found by summing the digits of the integer. If the resulting value is a single digit then that digit is the digital root. If the resulting value contains two or more digits, those digits are summed and the process is repeated. This is continued as long as necessary to obtain a single digit.
For example, consider the positive integer 24. Adding the 2 and the 4 yields a value of 6. Since 6 is a single digit, 6 is the digital root of 24. Now consider the positive integer 39. Adding the 3 and the 9 yields 12. Since 12 is not a single digit, the process must be repeated. Adding the 1 and the 2 yeilds 3, a single digit and also the digital root of 39.

Input
The input file will contain a list of positive integers, one per line. The end of the input will be indicated by an integer value of zero.
Output
For each integer in the input, output its digital root on a separate line of the output
Sample Input
24
39
0
Sample Output
6
3
题意：输入一个数（可能十分庞大），求解其位数之和，若结果大于10，继续求解其位数之和，直至位数之和为大于0小于10的整数。
C语言代码：

include “stdio.h”

include “string.h”

int digital(int n) //拆分位数，并求和
{
int a, b=0;
while(n!=0)
{
a=n%10;
b+=a;
n/=10;
}
if(b>9) return digital(b);
else return b;
}
void turn(char *str,int *number,int len) //字符串数组，化为整数数组
{
while(len–) *number++ = *str++ - ‘0’;
}
int main()
{
char str[10001];
int i, len, sum, number[10001];
while(gets(str)&&strcmp(str,”0”)!=0)
{
sum=0;
len=strlen(str);
turn(str,number,len);
for(i=0;i