Fix the prepend method for the LinkedList (trekhleb#47)

* Fix LinkedList * Fix the prepend method for the LinkedList

Author: m-maksyutin

src/data-structures/linked-list/LinkedList.js

@@ -21,7 +21,13 @@ export default class LinkedList {
  */
  prepend(value) {
  // Make new node to be a head.
- this.head = new LinkedListNode(value, this.head);
+ const newNode = new LinkedListNode(value, this.head);
+ this.head = newNode;
+
+ // If there is no tail yet let's make new node a tail.
+ if (!this.tail) {
+ this.tail = newNode;
+ }
 
  return this;
  }

src/data-structures/linked-list/__test__/LinkedList.test.js

@@ -21,10 +21,14 @@ describe('LinkedList', () => {
  it('should prepend node to linked list', () => {
  const linkedList = new LinkedList();
 
- linkedList.append(1);
  linkedList.prepend(2);
+ expect(linkedList.head.toString()).toBe('2');
+ expect(linkedList.tail.toString()).toBe('2');
+
+ linkedList.append(1);
+ linkedList.prepend(3);
 
- expect(linkedList.toString()).toBe('2,1');
+ expect(linkedList.toString()).toBe('3,2,1');
  });
 
  it('should delete node by value from linked list', () => {