using System;

using System.Collections.Generic;

public class Program

{

    /* Stack of Plates: Imagine a (literal) stack of plates. If the stack gets too high, it might topple.

    Therefore, in real life, we would likely start a new stack when the previous stack exceeds some

    threshold. Implement a data structure SetOfStacks that mimics this. SetOfStacks should be

    composed of several stacks and should create a new stack once the previous one exceeds capacity.

    SetOfStacks. push() and SetOfStacks. pop() should behave identically to a single stack

    (that is, pop () should return the same values as it would if there were just a single stack).

    FOLLOW UP

    Implement a function popAt(int index) which performs a pop operation on a specific substack. */

    public class StackOfPlates{

        public class Plate{}

        private int threshold;

        private List<Stack<Plate>> plates;

        public StackOfPlates(int threshold){

            this.threshold = threshold;

            this.plates = new List<Stack<Plate>>();

        }

        public void Push(Plate plate){

            var lastPlatePilePos = plates.Count - 1;

            if (lastPlatePilePos == -1 || plates[lastPlatePilePos].Count == threshold){

                var newPile = new Stack<Plate>();

                newPile.Push(plate);

                plates.Add(newPile);

            }

        }

        public Plate Pop(){

            if (plates.Count == 0) return null;

            var lastPlatePile = plates[plates.Count - 1];

            var plate = lastPlatePile.Pop();

            if (lastPlatePile.Count == 0) plates.RemoveAt(plates.Count-1);

            return plate;

        }

        private void MoveBottomPlate(Stack<Plate> source, Stack<Plate> target){

            var temp = new Stack<Plate>();

            while(source.Count != 0){

                temp.Push(source.Pop());

            }

            target.Push(temp.Pop());

            while(temp.Count != 0){

                source.Push(target.Pop());

            }

        }

        public Plate PopAt(int index){

            if (index >= plates.Count) return null;

            var pileAtIndex = plates[index];

            var plate = pileAtIndex.Pop();

            for (var i=index+1; i<plates.Count; i++){

                MoveBottomPlate(plates[i], plates[i-1]);

            }

            if (plates[plates.Count - 1].Count == 0){

                plates.RemoveAt(plates.Count-1);

            }

            return plate;

        }

    }

    public static void Main()

    {

        Console.WriteLine("Hello World");

    }

}