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.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func rangeSumBST(root *TreeNode, low int, high int) int {
	var dfs func(*TreeNode) int
	dfs = func(root *TreeNode) (ans int) {
		if root == nil {
			return 0
		}
		x := root.Val
		if low <= x && x <= high {
			ans += x
		}
		if x > low {
			ans += dfs(root.Left)
		}
		if x < high {
			ans += dfs(root.Right)
		}
		return
	}
	return dfs(root)
}