Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) of all the values of the nodes in the tree.

KthSmallestInBst.kt

/*
Given the reference to a binary search tree, return the kth the smallest value in the tree.
* */
fun kthSmallest(root: TreeNode?, k: Int): Int {
    val nodeQue: Queue<TreeNode?> = LinkedList<TreeNode?>()
    nodeQue.offer(root)
    val values: HashSet<Int> = HashSet()
    while (nodeQue.isNotEmpty()) {
            val currentNode: TreeNode? = nodeQue.poll()
            if (currentNode != null) {
                values.add(currentNode.`val`)
            }
            if (currentNode?.left != null) {
                nodeQue.offer(currentNode.left)
            }
            if (currentNode?.right != null) {
                nodeQue.offer(currentNode.right)
            }
    }
    return values.elementAt(k - 1)
}