kt: add «2816. Double a Number Represented as a Linked List»

Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/double-a-number-represented-as-a-linked-list.kt

@@ -0,0 +1,76 @@
+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<Int>): 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<ListNode?, Int> {
+        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)))
+}