fixed doubly-linked-list reverse method omit change node's previous (trekhleb#257)

Author: jadestrong

src/data-structures/doubly-linked-list/DoublyLinkedList.js

@@ -243,9 +243,11 @@ export default class DoublyLinkedList {
     while (currNode) {
       // Store next node.
       nextNode = currNode.next;
+      prevNode = currNode.previous;
 
       // Change next node of the current node so it would link to previous node.
       currNode.next = prevNode;
+      currNode.previous = nextNode;
 
       // Move prevNode and currNode nodes one step forward.
       prevNode = currNode;

src/data-structures/doubly-linked-list/__test__/DoublyLinkedList.test.js

@@ -236,22 +236,43 @@ describe('DoublyLinkedList', () => {
     linkedList
       .append(1)
       .append(2)
-      .append(3);
+      .append(3)
+      .append(4);
 
-    expect(linkedList.toString()).toBe('1,2,3');
+    expect(linkedList.toString()).toBe('1,2,3,4');
     expect(linkedList.head.value).toBe(1);
-    expect(linkedList.tail.value).toBe(3);
+    expect(linkedList.tail.value).toBe(4);
 
     // Reverse linked list.
     linkedList.reverse();
-    expect(linkedList.toString()).toBe('3,2,1');
-    expect(linkedList.head.value).toBe(3);
+    expect(linkedList.toString()).toBe('4,3,2,1');
+
+    expect(linkedList.head.previous).toBeNull();
+    expect(linkedList.head.value).toBe(4);
+    expect(linkedList.head.next.value).toBe(3);
+    expect(linkedList.head.next.next.value).toBe(2);
+    expect(linkedList.head.next.next.next.value).toBe(1);
+
+    expect(linkedList.tail.next).toBeNull();
     expect(linkedList.tail.value).toBe(1);
+    expect(linkedList.tail.previous.value).toBe(2);
+    expect(linkedList.tail.previous.previous.value).toBe(3);
+    expect(linkedList.tail.previous.previous.previous.value).toBe(4);
 
     // Reverse linked list back to initial state.
     linkedList.reverse();
-    expect(linkedList.toString()).toBe('1,2,3');
+    expect(linkedList.toString()).toBe('1,2,3,4');
+
+    expect(linkedList.head.previous).toBeNull();
     expect(linkedList.head.value).toBe(1);
-    expect(linkedList.tail.value).toBe(3);
+    expect(linkedList.head.next.value).toBe(2);
+    expect(linkedList.head.next.next.value).toBe(3);
+    expect(linkedList.head.next.next.next.value).toBe(4);
+
+    expect(linkedList.tail.next).toBeNull();
+    expect(linkedList.tail.value).toBe(4);
+    expect(linkedList.tail.previous.value).toBe(3);
+    expect(linkedList.tail.previous.previous.value).toBe(2);
+    expect(linkedList.tail.previous.previous.previous.value).toBe(1);
   });
 });