if p points to space, does it mean p is a null pointer?

If p points to anything then it cannot be a null pointer because it pointed to something!

meow meow

thanks for the feedback, there was a function name written wrong. now everything should be fine

@meonu, because x can become very large given large num

meow

oh i see! Thank you!

In cases ilke this, I mostly put the parentheses in to remind myself about the precedence of the '' (in partucular).

Thank you so much for your feedback!!! I've learned a lot!!

In the random tests arr2 is the expected solution, arr is the result of your DuplicateEncoder function.
!strcmp(arr, arr2) is false if the two don't match.

I just submitted my solution successfully, and also didn't see an obvious error in the tests.
If your problem persists, please post your solution here (and make sure to mark it as "having spoiler content").

There are several problems with your approach:

• malloc(strlen(str)) doesn't allocate enough space for the string and the terminating '\0'.
• When comparing a p[i] and p[j] you only convert the first one to lowercase.
  DuplicateEncoder("aA") and DuplicateEncoder("AA") should both return "))".
• If a character appears only once *(res + i) accesses unknown data, possibly even a ')'.

Also, why the complicated memset(res + i, ')', 1); instead of res[i] = ')';?

Did you modify the argument instead of allocating space for the result?