[Solved] 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 → …

Question

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

Python Solution

# 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:
        """
        Do not return anything, modify head in-place instead.
        """
        if head == None:
            return []
        slow = head
        fast = head.next

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

        second_list = slow.next
        slow.next = None

        prev = None
        curr = second_list

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

        second_list = prev
        first_list = head

        while second_list:
            temp1 = first_list.next
            temp2 = second_list.next
            second_list.next = None
            first_list.next = second_list
            second_list.next = temp1
            first_list = first_list.next.next
            second_list = temp2

        return head