kt: add «2487. Remove Nodes From Linked List»

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

kt/remove-nodes-from-linked-list.kt

@@ -0,0 +1,71 @@
+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 removeNodes(head: ListNode?): ListNode? = removeNodesRec(head).first
+
+    private fun removeNodesRec(head: ListNode?): Pair<ListNode?, Int> {
+        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)))
+}