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# 0669. Trim a Binary Search Tree

<https://leetcode.com/problems/trim-a-binary-search-tree>

## Description

Given the `root` of a binary search tree and the lowest and highest boundaries as `low` and `high`, trim the tree so that all its elements lies in `[low, high]`. Trimming the tree should **not** change the relative structure of the elements that will remain in the tree (i.e., any node's descendant should remain a descendant). It can be proven that there is a **unique answer**.

Return *the root of the trimmed binary search tree*. Note that the root may change depending on the given bounds.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/09/09/trim1.jpg)

```
**Input:** root = [1,0,2], low = 1, high = 2
**Output:** [1,null,2]
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/09/09/trim2.jpg)

```
**Input:** root = [3,0,4,null,2,null,null,1], low = 1, high = 3
**Output:** [3,2,null,1]
```

**Example 3:**

```
**Input:** root = [1], low = 1, high = 2
**Output:** [1]
```

**Example 4:**

```
**Input:** root = [1,null,2], low = 1, high = 3
**Output:** [1,null,2]
```

**Example 5:**

```
**Input:** root = [1,null,2], low = 2, high = 4
**Output:** [2]
```

**Constraints:**

* The number of nodes in the tree in the range `[1, 104]`.
* `0 <= Node.val <= 104`
* The value of each node in the tree is **unique**.
* `root` is guaranteed to be a valid binary search tree.
* `0 <= low <= high <= 104`

## ac

```java
class Solution {
    public TreeNode trimBST(TreeNode root, int L, int R) {
        if (root == null) return root;

        if (root.val < L) return trimBST(root.right, L, R);
        else if (root.val > R) return trimBST(root.left, L, R);

        root.left = trimBST(root.left, L, R);
        root.right = trimBST(root.right, L, R);

        return root;
    }
}

/*
recursively process left/right
*/
```