# Convert Sorted List to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of\_every\_node never differ by more than 1.

## **Example**

```
Given the sorted array: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5
```

## Note

两种input，要么是原生数组/列表，要么是链表。链表要麻烦一些

本质还是寻找中点，divide and conquer左右两边

base case：left > right: return null

## Code

```java
class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        if (nums == null || nums.length == 0) return null;
        return helper(nums, 0, nums.length - 1);
    }

    public TreeNode helper(int[] nums, int left, int right) {
        if (left > right) return null;
        int mid = (right - left) / 2 + left;
        TreeNode node = new TreeNode(nums[mid]);
        node.left  = helper(nums, left,    mid - 1);
        node.right = helper(nums, mid + 1, right);
        return node;
    }
```

```java
class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        if (head == null) return null;
        return helper(head, null);
    }

    public TreeNode helper(ListNode head, ListNode tail) {
        if (head == tail) return null;        

        ListNode slow = head;
        ListNode fast = head;

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

        TreeNode node = new TreeNode(slow.val);
        node.left  = helper(head, slow);
        node.right = helper(slow.next, tail);
        return node;
    }
}
```


---

# 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://luj.gitbook.io/code/tree/7-serialize-and-deserialize/convert-sorted-array-to-binary-search-tree.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.