653. Two Sum IV - Input is a BST

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

    private boolean dfs(TreeNode root, HashSet<Integer> set, int k)
    {
        if (root == null) return false;
        if (set.contains(k-root.val)) return true;
        set.add(root.val);
        return dfs(root.left,set,k) || dfs(root.right,set,k);
    }
    
    public boolean findTarget(TreeNode root, int k) {
        HashSet<Integer> set = new HashSet<>();
        return dfs(root,set,k);        
    }