I must revisit here to say thank you, it has opened my mind about regexes, you links are really helpful!
Keep up the good work!

I am very confused about the linked list thingy. Can you show me the code for preloaded function so I can try it on DEV C++?
Many thanks!

I passed 13/13 java tests and it said failed 0 but there is no SUBMIT button.
The same goes for my C solution, why is that?

I encountered this problem though it worked well on my DEV C++. I googled some and they all set about "free" function using malloc but I made use of none in my code. Can anyone tell me what's the problem here? Thank you!
*** Error in `/home/codewarrior/solution': munmap_chunk(): invalid pointer: 0x00007ffde2f0ee40 ***

You can't imagine how shocked I was upon seeing your solution after racking my brain for a fomula 😂😂😂

So this is all guessing or there is deeper logic to it?

Me too, I am slow at programming, I even unlocked the solution and did not really understand!