# 0143. Reorder List

<https://leetcode.com/problems/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:**

![](https://assets.leetcode.com/uploads/2021/03/04/reorder1linked-list.jpg)

```
**Input:** head = [1,2,3,4]
**Output:** [1,4,2,3]
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2021/03/09/reorder2-linked-list.jpg)

```
**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`

## ac

```java
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public void reorderList(ListNode head) {
        // corner cases
        if (head == null || head.next == null || head.next.next == null) return;

        // find mid, O(n)
        ListNode fast = head, slow = head;
        while (fast.next != null && fast.next.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }

        // reverese second half, O(n/2)
        ListNode l2 = slow.next;
        slow.next = null;
        ListNode prev = null;
        while (l2 != null ){
            ListNode tmp = l2;
            l2 = l2.next;
            tmp.next = prev;
            prev = tmp;
        }
        l2 = prev;

        // merge O(n)
        ListNode l1 = head.next;
        ListNode ranger = head;
        while (l1 != null) {
            ranger.next = l2;
            l2 = l2.next;
            ranger = ranger.next;

            ranger.next = l1;
            l1 = l1.next;
            ranger = ranger.next;
        }
        ranger.next = l2;
    }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://jaywin.gitbook.io/leetcode/solutions/0143-reorder-list.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.