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