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Exercise 3

Implement pop and write some test code to ensure that it's working.

Solution

The pop method returns null if the stack is empty, otherwise it returns the topmost node. This means that we need to keep a reference to this top node to return its value later.

Let's first see how popping elements from the stack would look visually. We'll continue where we left off in the previous image, with the stack containing three elements.

myStack.pop()

myStack.pop()

myStack.pop()

myStack.pop()

// some code above

pop() {
  if (!this.top) return null;

  const poppedNode = this.top;

  this.top = this.top.next;

  return poppedNode.val;
}

Finally, here's all of our code together:

class ListNode {
  constructor(val = 0, next = null) {
    this.val = val;
    this.next = next;
  }
}

class Stack {
  constructor() {
    this.top = null;
  }

  peek() {
    return this.top ? this.top.val : null;
  }

  push(value) {
    this.top = new ListNode(value, this.top);
  }

  pop() {
    if (!this.top) return null;

    const poppedNode = this.top;

    this.top = this.top.next;

    return poppedNode.val;
  }
}

const myStack = new Stack();
myStack.push(1);
console.log('Top element:', myStack.peek());  // logs 'Top element: 1'
myStack.push(2);
console.log('Top element:', myStack.peek());  // logs 'Top element: 2'
myStack.push(3);
console.log('Top element:', myStack.peek());  // logs 'Top element: 3'
myStack.pop();
console.log('Top element after pop:', myStack.peek());  // logs 'Top element after pop: 2'
myStack.pop();
console.log('Top element after pop:', myStack.peek());  // logs 'Top element after pop: 1'
myStack.pop();
console.log('Peek on empty stack:', myStack.peek());    // logs 'Peek on empty stack: null'

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