Medium
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
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def rotate_last(self, head):
node = head
prev = None
while node:
temp = node.next
node.next = prev
prev = node
node = temp
#self.show(prev)
return prev
def reorderList(self, head: Optional[ListNode]) -> None:
"""
Do not return anything, modify head in-place instead.
"""
node = head
slow, fast = node, node
while fast and fast.next:
slow = slow.next
fast = fast.next.next
end_head = self.rotate_last(slow.next)
slow.next = None
start = head
while start and end_head:
start_next = start.next
end_next = end_head.next
start.next = end_head
end_head.next = start_next
start = start_next
end_head = end_next
return head