public List<Integer> closestKValues(TreeNode root, double target, int k) {

LinkedList<Integer> res = new LinkedList<>();

helper(root, target, k, res);

return res;

}

private void helper(TreeNode root, double target, int k, LinkedList<Integer> res) {

if (root == null) return;

helper(root.left, target, k, res);

res.addLast(root.val);

helper(root.right, target, k, res);

cut(target, k, res);

}

private void cut(double target, int k, LinkedList<Integer> res) {

if (res.size() > k) {

if (Math.abs(res.getFirst() * 1.0 - target) >= Math.abs(res.getLast() * 1.0 - target)) {

res.removeFirst();

} else {

res.removeLast();

}

}

}