leetcode刷题笔记(python3)--143. Reorder List

143. Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You may not modify the values in the list’s nodes, only nodes itself may be changed.

Example 1:

Given 1->2->3->4, reorder it to 1->4->2->3.
Example 2:

Given 1->2->3->4->5, reorder it to 1->5->2->4->3.

计划是遍历 反转 插入，这个顺序去做，时间复杂度基本就是O(n)了

func reorderList(head *ListNode)  {
	if head == nil || head.Next == nil {
		return
	}
	fast, slow := head, head
	for fast.Next != nil && fast.Next.Next != nil {
		fast = fast.Next.Next
		slow = slow.Next
	}
	mid := slow.Next
	slow.Next = nil
	tail := mid
	var preNode *ListNode = nil
	for tail != nil {
		n := tail.Next
		tail.Next = preNode
		preNode = tail
		tail = n
	}
	//reverse
	for head != nil && preNode != nil {
		n := head.Next
		head.Next = preNode
        preNode = preNode.Next
		head.Next.Next = n
		head = n
	}
}

来源：CSDN

作者：圆滚滚coder

链接：https://blog.csdn.net/weixin_44555304/article/details/104420830