143. Reorder List

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

L₀ → L₁ → … → L_{n - 1} → L_n

Reorder the list to be on the following form:

L₀ → L_n → L₁ → L_{n - 1} → L₂ → L_{n - 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

Solutions (Python)

1. 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: Optional[ListNode]) -> None:
        """
        Do not return anything, modify head in-place instead.
        """
        length = 0
        curr = head
        while curr is not None:
            length += 1
            curr = curr.next

        prev = None
        curr = head
        for _ in range((length + 1) // 2):
            prev = curr
            curr = curr.next

        prev.next = None
        prev = None
        for _ in range(length // 2):
            temp = curr
            curr = curr.next
            temp.next = prev
            prev = temp

        curr0 = head
        curr1 = prev
        for _ in range(length // 2):
            prev0 = curr0
            prev1 = curr1
            curr0 = curr0.next
            curr1 = curr1.next
            prev0.next = prev1
            prev1.next = curr0