Purpose

Implement a Stack with push, pop, and peek methods.

• The push method should add an element to the "top" of the Stack.
• The pop method should remove the "top" element from the Stack.
• The peek method should return the "top" element of the Stack.

Approach

I decided to create a Node class to represent the elements in a Stack. The Node class would have a data field and a previous field that would reference another Node. By having this previous field one can "chain" Nodes together.

For my Stack class, I decided to set the head (i.e. the Node at the "top" of the Stack) value to null on initialization. I also decided to throw Exceptions when calling peek or pop for an empty Stack (i.e. when the head value is null).

I could see an argument to not throw Exceptions but rather return null for peek or nothing for pop (since it's void). However, null is a valid value for an Object, and since a Stack is made up of Objects, I think returning null when calling peek for an empty Stack could be confusing.

public class Node<T> {
  T data;
  Node<T> previous;

  public Node(T data, Node<T> previous) {
    this.data = data;
    this.previous = previous;
  }

  public T getData() {
    return data;
  }

  public Node<T> getPrevious() {
    return previous;
  }
}

public class Stack<T> {
  Node<T> head = null;

  public Stack() {
  }

  public T peek() {
    if (this.head == null) {
      throw new RuntimeException("Unable to peek element for an empty stack");
    }

    return this.head.getData();
  }

  public void push(T data) {
    this.head = new Node<>(data, this.head);
  }

  public void pop() {
    if (this.head == null) {
      throw new RuntimeException("Unable to remove element from an empty stack");
    }

    this.head = this.head.getPrevious();
  }