Complete hasCycle in key-concepts.js

key-concepts.js

@@ -782,4 +782,51 @@ var getElementsByClassName = function (className, element) {
   };
 
   return inner(element, []);
+};
+
+/*
+ * Assignment: Write a function that returns true if a linked list contains a cycle, or false if it terminates somewhere
+ *
+ * Explanation:
+ *
+ * Generally, we assume that a linked list will terminate in a null next pointer, as follows:
+ *
+ * A -> B -> C -> D -> E -> null
+ *
+ * A 'cycle' in a linked list is when traversing the list would result in visiting the same nodes over and over
+ * This is caused by pointing a node in the list to another node that already appeared earlier in the list. Example:
+ *
+ * A -> B -> C
+ *      ^    |
+ *      |    v
+ *      E <- D
+ *
+ * Example code:
+ *
+ * var nodeA = Node('A');
+ * var nodeB = nodeA.next = Node('B');
+ * var nodeC = nodeB.next = Node('C');
+ * var nodeD = nodeC.next = Node('D');
+ * var nodeE = nodeD.next = Node('E');
+ * hasCycle(nodeA); // => false
+ * nodeE.next = nodeB;
+ * hasCycle(nodeA); // => true
+ *
+ * Constraint 1: Do this in linear time
+ * Constraint 2: Do this in constant space
+ * Constraint 3: Do not mutate the original nodes in any way
+*/
+
+var hasCycle = function (node) {
+  var first  = node;
+  var second = node;
+
+  while (first && second) {
+    if (first === second) return true;
+
+    first = node.next;
+    second = node.next.next;
+  }
+
+  return false;
 };