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
Python3
# 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.
"""
slow, fast = head, head
while fast and fast.next:
slow = slow.next
fast = fast.next.next
def reverseLL(curr):
prev = None
while curr:
nxt = curr.next
curr.next = prev
prev = curr
curr = nxt
return prev
head2 = reverseLL(slow.next)
slow.next = None
while head and head2:
temp1 = head.next
temp2 = head2.next
head2.next = head.next
head.next = head2
head = temp1
head2 = temp2