implemented CycleDetectionII code in LinkedList (TheAlgorithms#1482)

* implemented CycleTectionII code * changes made per review by appgurueu * made the changes per review by appgurueu * changes made per review by appgurueu * did some changes * fixed the test file with prettier * Simplify code, renames for clarity --------- Co-authored-by: Lars Mueller <appgurulars@gmx.de>

Author: Akshay73c

DIRECTORY.md

@@ -82,6 +82,7 @@
  * **Linked-List**
  * [AddTwoNumbers](Data-Structures/Linked-List/AddTwoNumbers.js)
  * [CycleDetection](Data-Structures/Linked-List/CycleDetection.js)
+ * [CycleDetectionII](Data-Structures/Linked-List/CycleDetectionII.js)
  * [DoublyLinkedList](Data-Structures/Linked-List/DoublyLinkedList.js)
  * [MergeTwoSortedLinkedLists](Data-Structures/Linked-List/MergeTwoSortedLinkedLists.js)
  * [ReverseSinglyLinkedList](Data-Structures/Linked-List/ReverseSinglyLinkedList.js)

Data-Structures/Linked-List/CycleDetectionII.js

@@ -0,0 +1,57 @@
+/**
+ * A LinkedList based solution for finding the starting node of the cycle in a list.
+ * @returns the node where cycle begins in the linked list. If there is no cycle present, returns null.
+ * @see https://en.wikipedia.org/wiki/Cycle_detection
+ * @see https://leetcode.com/problems/linked-list-cycle-ii/
+ */
+
+function findCycleStart(head) {
+ let length = 0
+ let fast = head
+ let slow = head
+
+ while (fast !== null && fast.next !== null) {
+ fast = fast.next.next
+ slow = slow.next
+ if (fast === slow) {
+ length = cycleLength(slow)
+ break
+ }
+ }
+
+ if (length === 0) {
+ // If there is no cycle, return null.
+ return null
+ }
+
+ let ahead = head
+ let behind = head
+ // Move slow pointer ahead 'length' of cycle times
+ while (length > 0) {
+ ahead = ahead.next
+ length--
+ }
+
+ // Now move both pointers until they meet - this will be the start of cycle
+ while (ahead !== behind) {
+ ahead = ahead.next
+ behind = behind.next
+ }
+
+ // return the meeting node
+ return ahead
+}
+
+// head is a node on a cycle
+function cycleLength(head) {
+ // How long until we visit head again?
+ let cur = head
+ let len = 0
+ do {
+ cur = cur.next
+ len++
+ } while (cur != head)
+ return len
+}
+
+export { findCycleStart }

Data-Structures/Linked-List/test/CycleDetectionII.test.js

@@ -0,0 +1,39 @@
+import { findCycleStart } from '../CycleDetectionII'
+import { Node } from '../SinglyLinkedList'
+
+describe('Detect Cycle', () => {
+ it('no cycle', () => {
+ const head = new Node(1)
+ head.next = new Node(2)
+
+ expect(findCycleStart(head)).toBeNull()
+ })
+
+ it('simple cycle', () => {
+ const head = new Node(1)
+ head.next = new Node(2)
+ head.next.next = new Node(3)
+ head.next.next.next = head.next // Creates a cycle
+
+ expect(findCycleStart(head)).toBe(head.next)
+ })
+
+ it('long list with cycle', () => {
+ const head = new Node(1)
+ head.next = new Node(2)
+ head.next.next = new Node(3)
+ head.next.next.next = new Node(4)
+ head.next.next.next.next = new Node(5)
+ head.next.next.next.next.next = head.next.next // Cycle
+
+ expect(findCycleStart(head)).toBe(head.next.next)
+ })
+
+ it('cycle on last node', () => {
+ const head = new Node(1)
+ head.next = new Node(2)
+ head.next.next = head
+
+ expect(findCycleStart(head)).toBe(head)
+ })
+})