algorithm: find the middle of linked-list (#1096)

* Added Middle of linked-list implementation.

* Added Middle of LL function and tests

* Refactor: Added method in singly LL and its tests

* Refactor: Method name and inline test calls

* Use `!== null` instead of `!= null`

Co-authored-by: Lars Müller <34514239+appgurueu@users.noreply.github.com>

Data-Structures/Linked-List/SinglyLinkedList.js

@@ -6,7 +6,7 @@
 * a singly linked list.
 */
 
-// Methods - size, head, addLast, addFirst, addAt, removeFirst, removeLast, remove, removeAt, indexOf, isEmpty,  elementAt, get, clean
+// Methods - size, head, addLast, addFirst, addAt, removeFirst, removeLast, remove, removeAt, indexOf, isEmpty,  elementAt, findMiddle, get, clean
 
 class Node {
   constructor (data) {
@@ -193,6 +193,19 @@ class LinkedList {
     return removedNode.data
   }
 
+  // Returns a reference to middle node of linked list
+  findMiddle () {
+    // If there are two middle nodes, return the second middle node.
+    let fast = this.headNode
+    let slow = this.headNode
+
+    while (fast !== null && fast.next !== null) {
+      fast = fast.next.next
+      slow = slow.next
+    }
+    return slow
+  }
+
   // make the linkedList Empty
   clean () {
     this.headNode = null

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

@@ -164,6 +164,29 @@ describe('SinglyLinkedList', () => {
     expect(list.size()).toBe(1)
   })
 
+  it('Middle node of linked list', () => {
+    const list = new LinkedList()
+    list.addFirst(1)
+
+    // Middle node for list having single node
+    expect(list.findMiddle().data).toEqual(1)
+
+    list.addLast(2)
+    list.addLast(3)
+    list.addLast(4)
+    list.addLast(5)
+    list.addLast(6)
+    list.addLast(7)
+
+    // Middle node for list having odd number of nodes
+    expect(list.findMiddle().data).toEqual(4)
+
+    list.addLast(10)
+
+    // Middle node for list having even number of nodes
+    expect(list.findMiddle().data).toEqual(5)
+  })
+
   it('Check Iterator', () => {
     const list = new LinkedList()