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;
  }