public class Solution
extends Object
653 - Two Sum IV - Input is a BST\.
Easy
Given the `root` of a Binary Search Tree and a target number `k`, return _`true` if there exist two elements in the BST such that their sum is equal to the given target_.
**Example 1:**
**Input:** root = [5,3,6,2,4,null,7], k = 9
**Output:** true
**Example 2:**
**Input:** root = [5,3,6,2,4,null,7], k = 28
**Output:** false
**Constraints:**
* The number of nodes in the tree is in the range [1, 104].
* -104 <= Node.val <= 104
* `root` is guaranteed to be a **valid** binary search tree.
* -105 <= k <= 105