Trim a Binary Search Tree
<p>Given the <code>root</code> of a binary search tree and the lowest and highest boundaries as <code>low</code> and <code>high</code>, trim the tree so that all its elements lies in <code>[low, high]</code>. Trimming the tree should <strong>not</strong> change the relative structure of the elements that will remain in the tree (i.e., any node&#39;s descendant should remain a descendant). It can be proven that there is a <strong>unique answer</strong>.</p>

<p>Return <em>the root of the trimmed binary search tree</em>. Note that the root may change depending on the given bounds.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0600-0699/0669.Trim%20a%20Binary%20Search%20Tree/images/trim1.jpg" style="width: 450px; height: 126px;" />
<pre>
<strong>Input:</strong> root = [1,0,2], low = 1, high = 2
<strong>Output:</strong> [1,null,2]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0600-0699/0669.Trim%20a%20Binary%20Search%20Tree/images/trim2.jpg" style="width: 450px; height: 277px;" />
<pre>
<strong>Input:</strong> root = [3,0,4,null,2,null,null,1], low = 1, high = 3
<strong>Output:</strong> [3,2,null,1]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 10<sup>4</sup>]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 10<sup>4</sup></code></li>
	<li>The value of each node in the tree is <strong>unique</strong>.</li>
	<li><code>root</code> is guaranteed to be a valid binary search tree.</li>
	<li><code>0 &lt;= low &lt;= high &lt;= 10<sup>4</sup></code></li>
</ul>
Tree; Depth-First Search; Binary Search Tree; Binary Tree
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* trimBST(TreeNode* root, int low, int high) {
        if (!root) return root;
        if (root->val > high) return trimBST(root->left, low, high);
        if (root->val < low) return trimBST(root->right, low, high);
        root->left = trimBST(root->left, low, high);
        root->right = trimBST(root->right, low, high);
        return root;
    }
};