Update soluton: 141 (axross#118)

Author: Kohei Asai

solutions/linkedListCycle.test.ts

@@ -1,5 +1,7 @@
-import { createSinglyLinkedListNode } from "../testUtilities/LinkedList";
-import { SinglyLinkedListNode } from "../types/LinkedList";
+import {
+  createSinglyLinkedListNode,
+  getNthNode
+} from "../testUtilities/LinkedList";
 import hasCycle from "./linkedListCycle";
 
 describe("141. Linked List Cycle", () => {
@@ -10,22 +12,13 @@ describe("141. Linked List Cycle", () => {
   ]);
 
   for (const [[values, cycleStart], expected] of TEST_CASES) {
-    it(`returns ${expected} when called with [${values}] (cycle starts from ${cycleStart})`, () => {
-      const head = createSinglyLinkedListNode(values);
+    it(`returns ${expected} when called with [${values}] ${
+      cycleStart === -1 ? "(no cycle)" : `(cycle starts from ${cycleStart})`
+    }`, () => {
+      const head = createSinglyLinkedListNode(values)!;
 
-      let node = head;
-      let cycleStartNode: SinglyLinkedListNode<number> | null = null;
-
-      for (const i of values.keys()) {
-        if (i === cycleStart) {
-          cycleStartNode = node;
-        }
-
-        if (i === values.length - 1) {
-          node!.next = cycleStartNode;
-        }
-
-        node = node!.next;
+      if (cycleStart !== -1) {
+        getNthNode(head, -1).next = getNthNode(head, cycleStart);
       }
 
       expect(hasCycle(head)).toBe(expected);

solutions/linkedListCycle.ts

@@ -5,19 +5,15 @@ import { SinglyLinkedListNode } from "../types/LinkedList";
 export default function hasCycle<T>(
   head: SinglyLinkedListNode<T> | null
 ): boolean {
-  function traverse(
-    previous: SinglyLinkedListNode<T> | null,
-    node: SinglyLinkedListNode<T> | null
-  ): boolean {
-    if (node === null) return false;
-    if (node === head && previous !== null) return true;
+  let walker = head;
+  let runner = head ? head.next : null;
 
-    const next = node.next;
+  while (walker && runner) {
+    if (walker && walker === runner) return true;
 
-    node.next = previous;
-
-    return traverse(node, next);
+    walker = walker.next;
+    runner = runner.next ? runner.next.next : null;
   }
 
-  return traverse(null, head);
+  return false;
 }