# Convert Binary Search Tree to Sorted Circular Doubly-Linked List

[Leetcode 426: Convert Binary Search Tree to Sorted Doubly-Linked List](https://leetcode.com/problems/convert-binary-search-tree-to-sorted-doubly-linked-list/)

Convert a **Binary Search Tree** to a sorted **Circular
Doubly-Linked List** in place.

You can think of the left and right pointers as synonymous
to the predecessor and successor pointers in a doubly-linked
list. For a circular doubly linked list, the predecessor of
the first element is the last element, and the successor of
the last element is the first element.

We want to do the transformation **in place**. After the
transformation, the left pointer of the tree node should
point to its predecessor, and the right pointer should point
to its successor. You should return the pointer to the
smallest element of the linked list.

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## Example 1

![Original BST](https://assets.leetcode.com/uploads/2018/10/12/bstdlloriginalbst.png)

**Input:**  
`root = [4,2,5,1,3]`

![Returned DLL](https://assets.leetcode.com/uploads/2018/10/12/bstdllreturndll.png)

**Output:**  
`[1,2,3,4,5]`

**Explanation:**  
The figure below shows the transformed BST. The solid line
indicates the successor relationship, while the dashed line
means the predecessor relationship.

![Transformed BST](https://assets.leetcode.com/uploads/2018/10/12/bstdllreturnbst.png)

---

## Example 2

**Input:**  
`root = [2,1,3]`

**Output:**  
`[1,2,3]`

---

## Constraints

- The number of nodes in the tree is in the range
  `[0, 2000]`.
- `-1000 <= Node.val <= 1000`
- All the values of the tree are **unique**.