class Node {
  constructor(value){
    this.value = value
    this.next = null
  }
  setNext(next){
    this.next = next
  }
  getNext(){
    return this.next
  }
}

class LinkedList {
  constructor(){
    this.length = 0
    this.head = null
    this.tail = null
  }

  push(value){
    const newNode = new Node(value)

    if(this.length === 0) {
      this.head = this.tail = newNode
    }
    else {
      this.tail.setNext(newNode)
      this.tail = newNode
    }

    this.length++
  }

  pop(){
    if(this.length === 0) { return null }
    else {
      const oldTail = this.tail
      this.tail = this._findByIndex(this.length-2)
      this.tail.setNext(null)
      this.length--
      if(this.length === 0) { this.head = null }
      return oldTail.value
    }
  }

  get(index){
    return this._findByIndex(index).value
  }

  delete(index){
    let deletedNode
    if(index === 0) {
      deletedNode = this.head
      this.head = this.head.getNext()
    }
    else {
      const preNode = this._findByIndex(index-1)
      deletedNode = preNode.getNext()
      preNode.setNext(deletedNode.getNext())
    }
    this.length--
    return deletedNode.value
  }

  _findByIndex(index){
    let currentNode = this.head,
        counter = 0,
        max = this.length

    while (counter < max) {
      if(counter === index){
        return currentNode
      }
      currentNode = currentNode.getNext()
      counter++
    }
  }

  _findByNext(findNextNode){
    let currentNode = this.head
    while (currentNode.getNext() !== null) {
      currentNode = currentNode.getNext()
      if(currentNode.getNext() === findNextNode){
        return currentNode
      }
    }
  }
}