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653. Two Sum IV - Input is a BST
Description
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 <= 104rootis guaranteed to be a valid binary search tree.-105 <= k <= 105