ACM 333 - Recognizing Good ISBNs

#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main()
{
    char str[82];
    int i, numCount, s, e;
    while (gets(str))
    {
        if (strlen(str) == 0) { printf(" is incorrect.\n"); continue; }
        numCount = 0;
        int per = 0;
      int tsum = 0;
        int sum[9] = {0}, wrong = 0, d = 0, isX = 0, g = 0;
        for (s = 0; str[s] == ' ' || str[s] == '\t' || isalpha(str[s]) ; s ++) 
            if (str[s] == 'X') break; 
        for (e = strlen(str) - 1; str[e] == ' ' || str[e] == '\t' || isalpha(str[e]); e --)
            if (str[e] == 'X') break;  
        for (i = s; i <= e; i ++)
        {
            if (str[i] == '-') continue;
            else if (isdigit(str[i]))
            {
                numCount ++; 
                per += str[i] - '0';
          tsum += per;
            }
            else if (str[i] == 'X')
            {
                if (numCount != 9) { wrong = 1; break;}
                else
                {
                    per += 10;
              tsum += per;
                    numCount ++;
                }
            }
            else { wrong = 1; break;}
        }
        for (i = 0; str[i] == ' ' || str[i] == '\t'; i ++) ;
        s = i;
        for (i = strlen(str) - 1; str[i] == ' ' || str[i] == '\t'; i --) ;
        e = i;
        for (i = s; i <= e; i ++)
            putchar(str[i]);
        if (wrong) printf(" is incorrect.\n");
        else if (numCount >= 10 && tsum % 11 == 0) printf(" is correct.\n");
        else  printf(" is incorrect.\n");
                
    }
    return 0;
}

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