Range Sum of BST
<p>Given the <code>root</code> node of a binary search tree and two integers <code>low</code> and <code>high</code>, return <em>the sum of values of all nodes with a value in the <strong>inclusive</strong> range </em><code>[low, high]</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0900-0999/0938.Range%20Sum%20of%20BST/images/bst1.jpg" style="width: 400px; height: 222px;" />
<pre>
<strong>Input:</strong> root = [10,5,15,3,7,null,18], low = 7, high = 15
<strong>Output:</strong> 32
<strong>Explanation:</strong> Nodes 7, 10, and 15 are in the range [7, 15]. 7 + 10 + 15 = 32.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0900-0999/0938.Range%20Sum%20of%20BST/images/bst2.jpg" style="width: 400px; height: 335px;" />
<pre>
<strong>Input:</strong> root = [10,5,15,3,7,13,18,1,null,6], low = 6, high = 10
<strong>Output:</strong> 23
<strong>Explanation:</strong> Nodes 6, 7, and 10 are in the range [6, 10]. 6 + 7 + 10 = 23.
</pre>

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

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 2 * 10<sup>4</sup>]</code>.</li>
	<li><code>1 &lt;= Node.val &lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= low &lt;= high &lt;= 10<sup>5</sup></code></li>
	<li>All <code>Node.val</code> are <strong>unique</strong>.</li>
</ul>
Tree; Depth-First Search; Binary Search Tree; Binary Tree
/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function rangeSumBST(root: TreeNode | null, low: number, high: number): number {
    const dfs = (root: TreeNode | null): number => {
        if (!root) {
            return 0;
        }
        const { val, left, right } = root;
        let ans = low <= val && val <= high ? val : 0;
        if (val > low) {
            ans += dfs(left);
        }
        if (val < high) {
            ans += dfs(right);
        }
        return ans;
    };
    return dfs(root);
}
