Search Range in Binary Search Tree

Given two values k1 and k2 (where k1 < k2) and a root pointer to a Binary Search Tree. Find all the keys of tree in range k1 to k2. i.e. print all x such that k1<=x<=k2 and x is a key of given BST. Return all the keys in ascending order.

Example

If k1 =10and k2 =22, then your function should return[12, 20, 22].

    20
   /  \
  8   22
 / \
4   12

public ArrayList<Integer> searchRange(TreeNode root, int k1, int k2) {
    if(root == null) {
        return new ArrayList<>();
    }
    ArrayList<Integer> res = new ArrayList<Integer>();
    preorderSearch(root, k1, k2, res);
    Collections.sort(res);
    return res;
}
public void preorderSearch(TreeNode root, int k1, int k2, ArrayList<Integer> res) {
    if(root == null)
        return ;
    if(root.val >= k1 && root.val <= k2) {
        res.add(root.val);
    }
    preorderSearch(root.left, k1, k2, res);
    preorderSearch(root.right, k1, k2, res);
}