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`