implemented CycleDetectionII code in LinkedList (#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>

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)
+  })
+})