Add method, solution, and reference. (#32135)

* Add method, solution, and reference.

* fix: removed stub text

guide/english/certifications/coding-interview-prep/data-structures/reverse-a-doubly-linked-list/index.md

@@ -1,10 +1,69 @@
 ---
 title: Reverse a Doubly Linked List
 ---
+
 ## Reverse a Doubly Linked List
 
-This is a stub. <a href='https://github.com/freecodecamp/guides/tree/master/src/pages/certifications/coding-interview-prep/data-structures/reverse-a-doubly-linked-list/index.md' target='_blank' rel='nofollow'>Help our community expand it</a>.
+### Method:
+- Reverse the doubly linked list, and update previous and next variables for each member node accordingly.
+- Define privileged methods add() and reverse().
+- add() will find the end of the list and append new entries at this location.
+- reverse() will swap entries one pair at a time using a temporary variable.
 
-<a href='https://github.com/freecodecamp/guides/blob/master/README.md' target='_blank' rel='nofollow'>This quick style guide will help ensure your pull request gets accepted</a>.
+### Solution:
 
-<!-- The article goes here, in GitHub-flavored Markdown. Feel free to add YouTube videos, images, and CodePen/JSBin embeds  -->
+```js
+var Node = function(data, prev) {
+  this.data = data;
+  this.prev = prev;
+  this.next = null;
+};
+var DoublyLinkedList = function() {
+  this.head = null;
+  this.tail = null;
+  // change code below this line
+  this.add = function(element) {
+    let node = new Node(element, this.tail);
+    let currentNode = this.head;
+    let previousNode;
+
+    if (this.head === null) {
+      this.head = node;
+      this.tail = node;
+    } else {
+	    while (currentNode.next) {
+  	    previousNode = currentNode;
+        currentNode = currentNode.next;
+      }
+      node.prev = currentNode;
+      currentNode.next = node;
+      this.tail = node;
+    }
+  }; 
+  this.reverse = function() {
+    let temp = null;
+    let currentNode = this.head;
+    
+    if (this.head === null) {
+      return null;
+    }
+    
+    this.tail = currentNode;
+    
+    while (currentNode) {
+    	temp = currentNode.prev;
+      currentNode.prev = currentNode.next;
+      currentNode.next = temp;
+      currentNode = currentNode.prev;
+    }
+    
+    if (temp != null) {
+      this.head = temp.prev;
+    }
+  };
+  // change code above this line
+};
+```
+
+### Reference:
+- [Wikipedia](https://en.wikipedia.org/wiki/Doubly_linked_list)