Match sub-string within a string with tolerance of 1 character mismatch

Question

I was going through some Amazon interview questions on CareerCup.com, and I came across this interesting question which I haven\'t been able to figure out how to do. I have

Answer 1

This is slightly different than the earlier solution, but I was intrigued by the problem and wanted to give it a shot. Obviously optimize if desired, I just wanted a solution.

char *match(char *str, char *substr, int tolerance)
{
  if (! *substr) return str;
  if (! *str) return NULL;

  while (*str)
  {
    char *str_p;
    char *substr_p;
    char *matches_missing;
    char *matches_mismatched;

    str_p = str;
    substr_p = substr;

    while (*str_p && *substr_p && *str_p == *substr_p)
    {
      str_p++;
      substr_p++;
    }
    if (! *substr_p) return str;
    if (! tolerance)
    {
      str++;
      continue;
    }

    if (strlen(substr_p) <= tolerance) return str;

    /* missed due to a missing letter */
    matches_missing = match(str_p, substr_p + 1, tolerance - 1);
    if (matches_missing == str_p) return str;

    /* missed due to a mismatch of letters */
    matches_mismatched = match(str_p + 1, substr_p + 1, tolerance - 1);
    if (matches_mismatched == str_p + 1) return str;

    str++;
  }
  return NULL;
}