class ListNode(var `val`: Int) { var next: ListNode? = null } fun toListNode(vararg xs: Int): ListNode? { if (xs.isEmpty()) { return null } val dummy = ListNode(0) var node = dummy for (x in xs) { val next = ListNode(x) node.next = next node = next } return dummy.next } fun linkedListEquals(head: ListNode?, xs: List): Boolean { var node = head for ((i, x) in xs.withIndex()) { if (node == null) { println("[DEBUG] $x is expected at index $i in the linked list, but is not present") return false } else if (node.`val` != x) { println("[DEBUG] $x is expected at index $i in the linked list, but ${node.`val`} is present") return false } node = node.next } if (node != null) { println("[DEBUG] Linked list is longer than expected") } return node == null } class Solution { fun removeNodes(head: ListNode?): ListNode? = removeNodesRec(head).first private fun removeNodesRec(head: ListNode?): Pair { if (head == null) { return head to Int.MIN_VALUE } else if (head.next == null) { return head to head.`val` } val (next, rightMax) = removeNodesRec(head.next) var node = head if (head.`val` < rightMax) { node = next } else { node.next = next } return node to maxOf(head.`val`, rightMax) } } fun main() { val s = Solution() check(linkedListEquals(s.removeNodes(toListNode(5, 2, 13, 3, 8)), listOf(13, 8))) check(linkedListEquals(s.removeNodes(toListNode(1, 1, 1, 1)), listOf(1, 1, 1, 1))) }