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', () => {