Given a Binary Search Tree and a target number, return true if there exist two elements in the BST such that their sum is equal to the given target.

Example 1:

Input:

    5
   / \
  3   6
 / \   \
2   4   7

Target = 9


Output:
 True

Example 2:

Input:

    5
   / \
  3   6
 / \   \
2   4   7

Target = 28


Output:
 False

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean findTarget(TreeNode root, int k) {
        Set<Integer> set = new HashSet<>();

        return find(root, set, k);
    }

    private boolean find(TreeNode root, Set<Integer> set, int k){
        if (root == null) return false;

        if (set.contains(k - root.val)) return true;
        set.add(root.val);

        return find(root.left, set, k) || find(root.right, set, k);
    }
}