Add Linked List Midpoint solution

Author: marttp

kotlin/Fast and Slow Pointers/LinkedListMidpoint.kt

@@ -0,0 +1,16 @@
+/*
+    Definition of ListNode:
+    data class ListNode(var value: Int, var next: ListNode? = null)
+*/
+
+fun linkedListMidpoint(head: ListNode?): ListNode? {
+    var slow = head
+    var fast = head
+    // When the fast pointer reaches the end of the list, the slow
+    // pointer will be at the midpoint of the linked list.
+    while (fast != null && fast.next != null) {
+        slow = slow!!.next
+        fast = fast.next!!.next
+    }
+    return slow
+}