class ImmutableStack {
  constructor(arr) {
    this.arr = arr;
  }
  push(val) {
    let immutableStack = this.arr.slice();
    this.arr.push(val);
    return immutableStack;
  }
  pop() {
    let immutableStack = this.arr.slice();
    this.arr.pop();
    return immutableStack;
  }
}