Had my phone screening round at Coupang.

Question: Merge K sorted list of integers.

I gave the initial answer of storing current index for each list and picking the least value and incrementing the index value for that list.

Interviewer asked me to optimize it. After sometime he gave me the hint of not discarding all the elements iterated over but not selected for the current element.

I got the himt and gave the slution of using min heap. I was able to code both the solutions on time. However, I got rejected.