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 doubleIt(head: ListNode?): ListNode? { val dummyHead = ListNode(0) dummyHead.next = head val (newHead, carry) = doubleItRec(dummyHead) if (newHead!!.`val` > 0) { return newHead } return newHead.next } private fun doubleItRec(node: ListNode?): Pair { if (node == null) { return null to 0 } val (nextHead, carry) = doubleItRec(node.next) val current = carry + 2 * node.`val` val newNode = ListNode(current % 10) newNode.next = nextHead return newNode to current / 10 } } fun main() { val s = Solution() check(linkedListEquals(s.doubleIt(toListNode(1, 8, 9)), listOf(3, 7, 8))) check(linkedListEquals(s.doubleIt(toListNode(9, 9, 9)), listOf(1, 9, 9, 8))) check(linkedListEquals(s.doubleIt(toListNode(0)), listOf(0))) }