# 143. Reorder List

## Medium

***

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`

```python
# 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:
        left = head
        def recursion(right):
            nonlocal left
            if not right:
                return
            recursion(right.next)
            if left.next:
                temp = left.next
                left.next = right
                right.next = temp
                left = temp
            if left.next == right:
                left.next = None
        recursion(head)
        return left
            
```