Merge each two

If you can Merge Two Sorted Lists, then you can merge k by merging each two using a stack.

code

put lists into a stack

while stack has more than 2 lists

   merged = mergeTwoLists(stack.pop(), stack.pop())

   stack.push(merged)

Leetcode accpected the code above at Oct 2013. But, the code got a Time Limit Exceeded when I change ArrayList to List. (ArrayList is used on old version leetcode)

Reduce the time complexity to O(nlgn)

O(nlgn) reminds me that something is call MergeSort. So just write code like a MergeSort.

functino mergeKLists(lists)
  left  = mergeKLists(k / 2 lists)
  right = mergeKLists(k / 2 lists)

  return merge(left, right)