From 6f27113993cc53f988175ac6e3ca9368f3e1a1a9 Mon Sep 17 00:00:00 2001
From: Oleksii Trekhleb <trehleb@gmail.com>
Date: Sat, 8 Sep 2018 22:25:23 +0300
Subject: [PATCH] Add reverse() method for doubly linked list.

---
 .../doubly-linked-list/DoublyLinkedList.js    | 28 +++++++++++++++++++
 .../__test__/DoublyLinkedList.test.js         | 26 +++++++++++++++++
 2 files changed, 54 insertions(+)

diff --git a/src/data-structures/doubly-linked-list/DoublyLinkedList.js b/src/data-structures/doubly-linked-list/DoublyLinkedList.js
index 4afb404b..e66e016d 100644
--- a/src/data-structures/doubly-linked-list/DoublyLinkedList.js
+++ b/src/data-structures/doubly-linked-list/DoublyLinkedList.js
@@ -230,4 +230,32 @@ export default class DoublyLinkedList {
   toString(callback) {
     return this.toArray().map(node => node.toString(callback)).toString();
   }
+
+  /**
+   * Reverse a linked list.
+   * @returns {DoublyLinkedList}
+   */
+  reverse() {
+    let currNode = this.head;
+    let prevNode = null;
+    let nextNode = null;
+
+    while (currNode) {
+      // Store next node.
+      nextNode = currNode.next;
+
+      // Change next node of the current node so it would link to previous node.
+      currNode.next = prevNode;
+
+      // Move prevNode and currNode nodes one step forward.
+      prevNode = currNode;
+      currNode = nextNode;
+    }
+
+    // Reset head and tail.
+    this.tail = this.head;
+    this.head = prevNode;
+
+    return this;
+  }
 }
diff --git a/src/data-structures/doubly-linked-list/__test__/DoublyLinkedList.test.js b/src/data-structures/doubly-linked-list/__test__/DoublyLinkedList.test.js
index 5e4e81ea..ca6be64f 100644
--- a/src/data-structures/doubly-linked-list/__test__/DoublyLinkedList.test.js
+++ b/src/data-structures/doubly-linked-list/__test__/DoublyLinkedList.test.js
@@ -228,4 +228,30 @@ describe('DoublyLinkedList', () => {
     expect(node.value.customValue).toBe('test2');
     expect(linkedList.find({ value: 2, customValue: 'test5' })).toBeNull();
   });
+
+  it('should reverse linked list', () => {
+    const linkedList = new DoublyLinkedList();
+
+    // Add test values to linked list.
+    linkedList
+      .append(1)
+      .append(2)
+      .append(3);
+
+    expect(linkedList.toString()).toBe('1,2,3');
+    expect(linkedList.head.value).toBe(1);
+    expect(linkedList.tail.value).toBe(3);
+
+    // Reverse linked list.
+    linkedList.reverse();
+    expect(linkedList.toString()).toBe('3,2,1');
+    expect(linkedList.head.value).toBe(3);
+    expect(linkedList.tail.value).toBe(1);
+
+    // Reverse linked list back to initial state.
+    linkedList.reverse();
+    expect(linkedList.toString()).toBe('1,2,3');
+    expect(linkedList.head.value).toBe(1);
+    expect(linkedList.tail.value).toBe(3);
+  });
 });