"""
143. Reorder List
Difficulty: Medium
https://leetcode.com/problems/reorder-list/

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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 * 10^4].

        • 1 <= Node.val <= 1000
"""


# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reorderList(self, head: ListNode | None) -> None:
        # Find middle
        slow, fast = head, head.next
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next

        # Reverse second half
        second = slow.next
        prev = slow.next = None
        while second:
            tmp = second.next
            second.next = prev
            prev = second
            second = tmp

        # merge two halfs
        first, second = head, prev
        while second:
            tmp1, tmp2 = first.next, second.next
            first.next = second
            second.next = tmp1
            first, second = tmp1, tmp2