#143

Reorder List

specialist · 635 · lc medium +30 · failed · 64.7% accepted · 12,470 likes · top 68%

Description

You are given the head of a singly linked-list arranged as:

L0 → L1 → … → Ln - 1 → Ln

Rearrange the nodes into the following alternating pattern:

L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 → …

Node values must not be changed; only the node connections may be modified.

Example 1:

L0 → L1 → … → Ln - 1 → Ln

Example 2:

L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 → …

Example 3:

Input: head = [1,2,3,4]

Output: [1,4,2,3]

Example 4:

Input: head = [1,2,3,4,5]

Output: [1,5,2,4,3]

Code

# 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.
        """

# 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.
        """