Write the function htoi(s), which converts a string of hexadecimal digits(including an optional x 0r 0X) into its equivalent integer value.The allowable digits are 0 through 9, a through f, and A through F.

#define    YES       1
#define    NO        0

/*htoi:convert hexadecimal string s to integer    */
int htoi(char s[]){
    int hexdigit, i, inhex, n;

    i =0;
    if (s[i]  == '0'){   /* skip optional 0x or 0X */
        ++i;
        if (s[i] == 'x' || s[i] == 'X')
             ++i;
    }
     n = 0;          /* integer value to be returned */
     inhex = YES     /*assume valid hexadecimal digit */
     for(; inhex == YES; ++i) {
          if (s[i] >= '0'  && s[i]  <= '9')
              hexdigit = s[i] - '0';
          else if (s[i] >= 'a' && s[i] <= 'f')
             hexdigit = s[i] - 'a' + 10;
          else if (s[i] >= 'A' && s[i] <= 'F')
            hexdigit = s[i]  - 'A' + 10;
          else
              inhex = NO;    /* not a valid hexadecimal digit */
          if (inhex == YES)
              n = 16 * n   + hexdigit;
     }
    return n;
}