I saw many solution have 0k, what does it means, how can a code doesn’t take memory, isn’t it practically incorrect?
Please explain me, what does 0K exactly mean?
It means exactly what you expect, but you have to realize that it is in KB instead of B, so when you use 2 ints it’s 4B and it’s 0.004KB, that’s why 
I’m not sure how it’s calculated when malloc is used, but I expect that if it’s freed it’s also 0KB.