给定一个单链表 L 的头节点 head ，单链表 L 表示为：
L0 → L1 → … → Ln - 1 → Ln
请将其重新排列后变为：
L0 → Ln → L1 → Ln - 1 → L2 → Ln - 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
先通过快慢指针找到链表中点，将链表划分为左右两部分。之后反转右半部分的链表，然后将左右两个链接依次连接即可。
# 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: ListNode) -> None:
"""
Do not return anything, modify head in-place instead.
"""
if head is None or head.next is None:
return
# 快慢指针找到链表中点
slow, fast = head, head.next
while fast and fast.next:
slow, fast = slow.next, fast.next.next
# cur 指向右半部分链表
cur = slow.next
slow.next = None
# 反转右半部分链表
pre = None
while cur:
t = cur.next
cur.next = pre
pre, cur = cur, t
cur = head
# 此时 cur, pre 分别指向链表左右两半的第一个节点
while pre:
t = pre.next
pre.next = cur.next
cur.next = pre
cur, pre = pre.next, t/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode() {}
* ListNode(int val) { this.val = val; }
* ListNode(int val, ListNode next) { this.val = val; this.next = next; }
* }
*/
class Solution {
public void reorderList(ListNode head) {
if (head == null || head.next == null) {
return;
}
ListNode slow = head;
ListNode fast = head.next;
while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
}
ListNode cur = slow.next;
slow.next = null;
ListNode pre = null;
while (cur != null) {
ListNode t = cur.next;
cur.next = pre;
pre = cur;
cur = t;
}
cur = head;
while (pre != null) {
ListNode t = pre.next;
pre.next = cur.next;
cur.next = pre;
cur = pre.next;
pre = t;
}
}
}/**
* Definition for singly-linked list.
* public class ListNode {
* public int val;
* public ListNode next;
* public ListNode(int val=0, ListNode next=null) {
* this.val = val;
* this.next = next;
* }
* }
*/
public class Solution {
public void ReorderList(ListNode head) {
if (head == null || head.next == null)
{
return;
}
ListNode slow = head;
ListNode fast = head.next;
while (fast != null && fast.next != null)
{
slow = slow.next;
fast = fast.next.next;
}
ListNode cur = slow.next;
slow.next = null;
ListNode pre = null;
while (cur != null)
{
ListNode t = cur.next;
cur.next = pre;
pre = cur;
cur = t;
}
cur = head;
while (pre != null)
{
ListNode t = pre.next;
pre.next = cur.next;
cur.next = pre;
cur = pre.next;
pre = t;
}
}
}/**
* Definition for singly-linked list.
* type ListNode struct {
* Val int
* Next *ListNode
* }
*/
func reorderList(head *ListNode) {
if head == nil || head.Next == nil {
return
}
slow, fast := head, head.Next
for fast != nil && fast.Next != nil {
slow, fast = slow.Next, fast.Next.Next
}
cur := slow.Next
slow.Next = nil
var pre *ListNode
for cur != nil {
t := cur.Next
cur.Next = pre
pre, cur = cur, t
}
cur = head
for pre != nil {
t := pre.Next
pre.Next = cur.Next
cur.Next = pre
cur, pre = pre.Next, t
}
}/**
* Definition for singly-linked list.
* function ListNode(val, next) {
* this.val = (val===undefined ? 0 : val)
* this.next = (next===undefined ? null : next)
* }
*/
/**
* @param {ListNode} head
* @return {void} Do not return anything, modify head in-place instead.
*/
var reorderList = function (head) {
if (!head || !head.next) {
return;
}
let slow = head;
let fast = head.next;
while (fast && fast.next) {
slow = slow.next;
fast = fast.next.next;
}
let cur = slow.next;
slow.next = null;
let pre = null;
while (cur) {
const t = cur.next;
cur.next = pre;
pre = cur;
cur = t;
}
cur = head;
while (pre) {
const t = pre.next;
pre.next = cur.next;
cur.next = pre;
cur = pre.next;
pre = t;
}
};/**
* Definition for singly-linked list.
* class ListNode {
* val: number
* next: ListNode | null
* constructor(val?: number, next?: ListNode | null) {
* this.val = (val===undefined ? 0 : val)
* this.next = (next===undefined ? null : next)
* }
* }
*/
/**
Do not return anything, modify head in-place instead.
*/
function reorderList(head: ListNode | null): void {
const arr = [];
let node = head;
while (node.next != null) {
arr.push(node);
node = node.next;
}
let l = 0;
let r = arr.length - 1;
while (l < r) {
const start = arr[l];
const end = arr[r];
[end.next.next, start.next, end.next] = [start.next, end.next, null];
l++;
r--;
}
}/**
* Definition for singly-linked list.
* class ListNode {
* val: number
* next: ListNode | null
* constructor(val?: number, next?: ListNode | null) {
* this.val = (val===undefined ? 0 : val)
* this.next = (next===undefined ? null : next)
* }
* }
*/
/**
Do not return anything, modify head in-place instead.
*/
function reorderList(head: ListNode | null): void {
let slow = head;
let fast = head;
// 找到中心节点
while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
}
// 反转节点
let next = slow.next;
slow.next = null;
while (next != null) {
[next.next, slow, next] = [slow, next, next.next];
}
// 合并
let left = head;
let right = slow;
while (right.next != null) {
const next = left.next;
left.next = right;
right = right.next;
left.next.next = next;
left = left.next.next;
}
}