143. Reorder List

Medium

You are given the head of a singly linked-list. The list can be represented as:

L0 → L1 → … → Ln - 1 → Ln

Reorder the list to be on the following form:

L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 → …

You may not modify the values in the list's nodes. Only nodes themselves may be changed.

Example 1:

Input: head = [1,2,3,4]
Output: [1,4,2,3]

Example 2:

Input: head = [1,2,3,4,5]
Output: [1,5,2,4,3]

Constraints:

• The number of nodes in the list is in the range [1, 5 * 104].
• 1 <= Node.val <= 1000

Solution

class Solution:
    def reorderList(self, head: ListNode) -> None:
        def splitAtMid(head: ListNode):
            prev = None
            slow = head
            fast = head

            while fast and fast.next:
                prev = slow
                slow = slow.next
                fast = fast.next.next

            prev.next = None

            return slow

        def reverse(head: ListNode) -> ListNode:
            prev = None
            curr = head

            while curr:
                next = curr.next
                curr.next = prev
                prev = curr
                curr = next

            return prev

        def merge(l1: ListNode, l2: ListNode) -> None:
            while l2:
                next = l1.next
                l1.next = l2
                l1 = l2
                l2 = next

        if not head or not head.next:
            return

        mid = splitAtMid(head)
        reversed = reverse(mid)
        merge(head, reversed)