Add Linked List Loop solutions

Author: marttp

kotlin/Fast and Slow Pointers/LinkedListLoop.kt

@@ -0,0 +1,19 @@
+/*
+    Definition of ListNode:
+    data class ListNode(var value: Int, var next: ListNode? = null)
+*/
+
+fun linkedListLoop(head: ListNode?): Boolean {
+    var slow = head
+    var fast = head
+    // Check both 'fast' and 'fast.next' to avoid null pointer
+    // exceptions when we perform 'fast.next' and 'fast.next.next'.
+    while (fast != null && fast.next != null) {
+        slow = slow!!.next
+        fast = fast.next!!.next
+        if (fast == slow) {
+            return true
+        }
+    }
+    return false
+}

kotlin/Fast and Slow Pointers/LinkedListLoopNaive.kt

@@ -0,0 +1,18 @@
+/*
+    Definition of ListNode:
+    data class ListNode(var value: Int, var next: ListNode? = null)
+*/
+
+fun linkedListLoopNaive(head: ListNode?): Boolean {
+    val visited = mutableSetOf<ListNode>()
+    var curr = head
+    while (curr != null) {
+        // Cycle detected if the current node has already been visited.
+        if (visited.contains(curr)) {
+            return true
+        }
+        visited.add(curr)
+        curr = curr.next
+    }
+    return false
+}