README_CN.md

143. 重排链表

给定一个单链表 L 的头节点 head ，单链表 L 表示为：

L₀ → L₁ → … → L_{n - 1} → L_n

请将其重新排列后变为：

L₀ → L_n → L₁ → L_{n - 1} → L₂ → L_{n - 2} → …

不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。

示例 1:

输入: head = [1,2,3,4]
输出: [1,4,2,3]

示例 2:

输入: head = [1,2,3,4,5]
输出: [1,5,2,4,3]

提示:

• 链表的长度范围为 [1, 5 * 104]
• 1 <= Node.val <= 1000

题解 (Python)

1. 题解

# 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