# 143. Reorder List

## Description

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

## Solutions

• /**
* 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 {
while (fast.next != null && fast.next.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;
}

while (pre != null) {
ListNode t = pre.next;
pre.next = cur.next;
cur.next = pre;
cur = pre.next;
pre = t;
}
}
}

• /**
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode() : val(0), next(nullptr) {}
*     ListNode(int x) : val(x), next(nullptr) {}
*     ListNode(int x, ListNode *next) : val(x), next(next) {}
* };
*/
class Solution {
public:
while (fast->next && fast->next->next) {
slow = slow->next;
fast = fast->next->next;
}

ListNode* cur = slow->next;
slow->next = nullptr;

ListNode* pre = nullptr;
while (cur) {
ListNode* t = cur->next;
cur->next = pre;
pre = cur;
cur = t;
}

while (pre) {
ListNode* t = pre->next;
pre->next = cur->next;
cur->next = pre;
cur = pre->next;
pre = t;
}
}
};

• # 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:
while fast.next and fast.next.next:
slow = slow.next
fast = fast.next.next

cur = slow.next
slow.next = None

pre = None
while cur:
t = cur.next
cur.next = pre
pre, cur = cur, t

while pre:
t = pre.next
pre.next = cur.next
cur.next = pre
cur, pre = pre.next, t


• /**
* type ListNode struct {
*     Val int
*     Next *ListNode
* }
*/
for fast.Next != nil && fast.Next.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
}

for pre != nil {
t := pre.Next
pre.Next = cur.Next
cur.Next = pre
cur, pre = pre.Next, t
}
}

• /**
* 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)
*     }
* }
*/

/**
*/
function reorderList(head: ListNode | null): void {

while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
}

let next = slow.next;
slow.next = null;
while (next != null) {
[next.next, slow, next] = [slow, next, next.next];
}

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;
}
}


• /**
* function ListNode(val, next) {
*     this.val = (val===undefined ? 0 : val)
*     this.next = (next===undefined ? null : next)
* }
*/
/**
*/
var reorderList = function (head) {
while (fast.next && fast.next.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;
}

while (pre) {
const t = pre.next;
pre.next = cur.next;
cur.next = pre;
cur = pre.next;
pre = t;
}
};


• /**
* 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 {
while (fast.next != null && fast.next.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;
}

while (pre != null) {
ListNode t = pre.next;
pre.next = cur.next;
cur.next = pre;
cur = pre.next;
pre = t;
}
}
}

• // Definition for singly-linked list.
// #[derive(PartialEq, Eq, Clone, Debug)]
// pub struct ListNode {
//   pub val: i32,
//   pub next: Option<Box<ListNode>>
// }
//
// impl ListNode {
//   #[inline]
//   fn new(val: i32) -> Self {
//     ListNode {
//       next: None,
//       val
//     }
//   }
// }
use std::collections::VecDeque;
impl Solution {
pub fn reorder_list(head: &mut Option<Box<ListNode>>) {
let mut tail = &mut head.as_mut().unwrap().next;
let mut deque = VecDeque::new();
}
let mut flag = false;
while !deque.is_empty() {
*tail = if flag { deque.pop_front().unwrap() } else { deque.pop_back().unwrap() };
tail = &mut tail.as_mut().unwrap().next;
flag = !flag;
}
}
}