Adding a solution for reversing a DoubleLinkedList. (#38427)

curriculum/challenges/english/08-coding-interview-prep/data-structures/reverse-a-doubly-linked-list.english.md

@@ -122,7 +122,35 @@ DoublyLinkedList.prototype = Object.assign(
 <section id='solution'>
 
 ```js
-// solution required
+  var Node = function(data, prev) {
+    this.data = data;
+    this.prev = prev;
+    this.next = null;
+  };
+  var DoublyLinkedList = function() {
+    this.head = null;
+    this.tail = null;
+
+    this.reverse = function() {
+      if (!this.head || !this.head.next) {
+        return this.head
+      }
+
+      let tail;
+      let temp;
+      let current = this.head;
+      while(current !== null) {
+        if(!tail) tail = current;
+        temp = current.prev;
+        current.prev = current.next;
+        current.next = temp;
+        current = current.prev;
+      }
+
+      this.head = temp.prev;
+      this.tail = tail
+    }
+  };
 ```
 
 </section>