class node {
  constructor(val) {
    this.val = val;
    this.next = null;

  }
}

class stack {
  constructor() {
    this.length = 0;
    this.first = null;
    this.prev = null;
    this.c = null;

    this.last = null;



  }
  push(val) {
    var nod = new node(val);
    if (this.length == 0) {
      this.first = nod;
      this.last = nod;
      ++this.length;

    }
    else if (this.length > 0) {
      nod.next = this.first;
      this.first = nod;

    }
    return this
  }

  pop() {
    console.log(this.first);
    if (!this.first.next) {
      return;

    }
    this.first = this.first.next;
    --this.length;


  }
}
const stackv = new stack();
stackv.push(1);
stackv.push(2);
stackv.push(3);
stackv.push(4);

stackv.pop()
//stack.show();

오늘의 교훈 생각이 안나면 반대 방향으로도 해보기

this.c.next= this.first 식으로 하면 로직이 복잡해짐