Add comments to Stack code.

Author: trekhleb

src/data-structures/stack/Stack.js

@@ -2,13 +2,17 @@ import LinkedList from '../linked-list/LinkedList';
 
 export default class Stack {
   constructor() {
+    // We're going to implement Queue based on LinkedList since this
+    // structures a quite similar. Compare push/pop operations of the Stack
+    // with append/deleteTail operations of LinkedList.
     this.linkedList = new LinkedList();
   }
 
   /**
    * @return {boolean}
    */
   isEmpty() {
+    // The queue is empty in case if its linked list don't have tail.
     return !this.linkedList.tail;
   }
 
@@ -17,23 +21,29 @@ export default class Stack {
    */
   peek() {
     if (this.isEmpty()) {
+      // If linked list is empty then there is nothing to peek from.
       return null;
     }
 
+    // Just read the value from the end of linked list without deleting it.
     return this.linkedList.tail.value;
   }
 
   /**
    * @param {*} value
    */
   push(value) {
+    // Pushing means to lay the value on top of the stack. Therefore let's just add
+    // new value at the end of the linked list.
     this.linkedList.append(value);
   }
 
   /**
    * @return {*}
    */
   pop() {
+    // Let's try to delete the last node from linked list (the tail).
+    // If there is no tail in linked list (it is empty) just return null.
     const removedTail = this.linkedList.deleteTail();
     return removedTail ? removedTail.value : null;
   }