用C实现字符串replace函数, 真累, 差点整不出来

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void replace(char* res, const char* oldstr, char* newstr)
{
if (res == NULL || oldstr == NULL || newstr == NULL)
return;

//alloc memory who bigger
int i = 0;
int oldlen = 0, newlen = 0, difflen = 0;
oldlen = strlen(oldstr);
newlen = strlen(newstr);

if (newlen > oldlen)
difflen = newlen - oldlen;
else
difflen = oldlen - newlen;

char *pt;
while ((pt = strstr(res, oldstr)) != NULL)
{
  i = 0;
  if (oldlen > newlen) //new string shorten
  {
   for (i=0; i<newlen; i++)
   {
    *pt = newstr[i];
    pt++;
   }

   while (*(pt+difflen) != '/0')
   {
    *pt = *(pt+difflen);
    pt++;
   }
   *pt = '/0';
  }
  else
  {
   char *p = (&res[strlen(res)]);
   for (p; p != pt; p--)
   {
    *p = *(p-difflen);
   }

   i = 0;
   while (i < newlen)
   {
    *pt = newstr[i];
    pt++;
    i++;
   }
  }
}
}

int main(int argc, char* argv[])
{
char a[128], b[128];

memset(a, 0, sizeof(a));
strcpy(a, "https://1.1.1.1/nhttps://2.2.2.2/nhttps://3.3.3.3");
printf("%s/n", a);
replace(a, "https://", "");
printf("%s/n", a);

printf("----------------/n");
memset(b, 0, sizeof(b));
strcpy(b, "http://1.1.1.1/nhttp://2.2.2.2/nhttp://3.3.3.3"); //
printf("%s/n", b);
replace(b, "http", "ftp");
printf("%s/n", b);
return 0;
}