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

 1     private boolean dfs(TreeNode root, HashSet<Integer> set, int k)
 2     {
 3         if (root == null) return false;
 4         if (set.contains(k-root.val)) return true;
 5         set.add(root.val);
 6         return dfs(root.left,set,k) || dfs(root.right,set,k);
 7     }
 8     
 9     public
 
 boolean findTarget(TreeNode root, int k) {
10         HashSet<Integer> set = new HashSet<>();
11         return dfs(root,set,k);        
12     }

653. Two Sum IV - Input is a BST