Sort List - Implementation

Here is the implementation:

function sortList(head):
    if head == null or head.next == null:
        return head

    // Find middle with slow/fast
    slow = head, fast = head.next
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next

    // Split
    mid = slow.next
    slow.next = null

    // Recurse and merge
    left = sortList(head)
    right = sortList(mid)
    return merge(left, right)

function merge(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

$O(n \log n)$ time, $O(\log n)$ stack space.