Merge k Sorted Lists - Implementation

Here is the implementation:

function mergeKLists(lists):
    if lists is empty:
        return null
    return mergeRange(lists, 0, lists.length - 1)

function mergeRange(lists, left, right):
    if left == right:
        return lists[left]

    mid = (left + right) / 2
    l1 = mergeRange(lists, left, mid)
    l2 = mergeRange(lists, mid + 1, right)
    return mergeTwoLists(l1, l2)

function mergeTwoLists(l1, l2):
    dummy = new ListNode()
    curr = dummy
    while l1 and l2:
        if l1.val <= l2.val:
            curr.next = l1
            l1 = l1.next
        else:
            curr.next = l2
            l2 = l2.next
        curr = curr.next
    curr.next = l1 or l2
    return dummy.next

$O(N \log k)$ time, $O(\log k)$ space for recursion.