143. Reorder List

Description

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

reorder-list.py

# 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.
        """
        p1, p2 = head, head.next
        while p2 and p2.next:
            p1 = p1.next
            p2 = p2.next.next

        def reverseLL(curr):
            prev = None

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

            return prev

        head2 = reverseLL(p1.next)
        p1.next = None

        while head and head2:
            tmp1 = head.next
            tmp2 = head2.next
            head2.next = head.next
            head.next = head2
            head = tmp1
            head2 = tmp2