http://uva.onlinejudge.org/external/113/11332.html

bpe10473.cpp

#include <cstdio>
#include <cstring>

int main (void)
{
    char s[11];

    while ( scanf("%s", s) != EOF  &&  s[0] != '0' )
    {
        while ( strlen(s) != 1 )
        {
            int F = 0;
            for ( int i = 0; i < (int)strlen(s); i++ )
            {
                F += s[i] - '0';
            }
            memset(s, '\0', 11);
            sprintf(s, "%d", F);
        }
        printf("%s\n", s);
    }

    return 0;
}