--refactor : refactor length function

Author: TheSTL

src/_DataStructures_/DoublyLinkedList/index.js

@@ -4,6 +4,7 @@ class Node {
     this.data = data;
     this.previous = previous;
     this.next = next;
+    this.length = 0;
   }
 }
 
@@ -21,20 +22,24 @@ class DoublyLinkedList {
     const newNode = new Node(value, this.head, this.head.next);
     this.head.next.previous = newNode;
     this.head.next = newNode;
+    this.length += 1;
   }
 
   addAtEnd(value) {
     const newNode = new Node(value, this.tail.previous, this.tail);
     this.tail.previous.next = newNode;
     this.tail.previous = newNode;
+    this.length += 1;
   }
 
   removeAtBeginning() {
     this.remove(this.head.next);
+    this.length -= 1;
   }
 
   removeAtEnd() {
     this.remove(this.tail.previous);
+    this.length -= 1;
   }
 
   remove(node) {
@@ -45,13 +50,7 @@ class DoublyLinkedList {
   }
 
   length() {
-    let address = this.head.next;
-    let count = 0;
-    while (address !== this.tail) {
-      count += 1;
-      address = address.next;
-    }
-    return count;
+    return this.length;
   }
 
   display() {