Fix missing "currentNode" variable declaration in data structures linked list challenges (English) (#35638)

* Fix missing variable declaration in data structures challenges (English)

* Fix duplicate currentNode var declaration.

curriculum/challenges/english/08-coding-interview-prep/data-structures/add-elements-at-a-specific-index-in-a-linked-list.english.md

@@ -64,7 +64,7 @@ function LinkedList() {
     if(head === null){
         head = node;
     } else {
-        currentNode = head;
+        var currentNode = head;
 
         while(currentNode.next){
             currentNode  = currentNode.next;

curriculum/challenges/english/08-coding-interview-prep/data-structures/remove-elements-from-a-linked-list-by-index.english.md

@@ -67,7 +67,7 @@ function LinkedList() {
     if(head === null){
         head = node;
     } else {
-        currentNode = head;
+        var currentNode = head;
 
         while(currentNode.next){
             currentNode  = currentNode.next;

curriculum/challenges/english/08-coding-interview-prep/data-structures/remove-elements-from-a-linked-list.english.md

@@ -46,7 +46,7 @@ tests:
 function LinkedList() {
   var length = 0;
   var head = null;
-  var currentNode = null;
+
   var Node = function(element){
     this.element = element;
     this.next = null;
@@ -65,7 +65,7 @@ function LinkedList() {
     if(head === null){
         head = node;
     } else {
-        currentNode = head;
+        var currentNode = head;
 
         while(currentNode.next){
             currentNode  = currentNode.next;

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

@@ -64,7 +64,7 @@ function LinkedList() {
     if(head === null){
         head = node;
     } else {
-        currentNode = head;
+        var currentNode = head;
 
         while(currentNode.next){
             currentNode  = currentNode.next;