stacks.js

// What is added last, is removed first.
class Node {
  constructor(value) {
    this.val = value
    this.next = null
  }
}

class Stack {
  constructor () {
    this.first = null
    this.last = null
    this.size = 0
  }

  push(val) {
    let newNode = new Node(val)
    if (!this.first) {
      this.first = newNode
      this.last = newNode
    } else {
      let temp = this.first
      this.first = newNode
      this.first.next = temp
    }
    this.size++
    return this.size
  }

  pop() {
    if (this.size === 0) return null
    
    let temp = this.first
    if (this.size === 1) {
      this.first = null
      this.last = null
    } 

    this.first = this.first.next
    this.length--
    return this.temp
  }
}

// Insertion is O(n)
// Removal is O(n)