using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        MaxStack stack = new MaxStack();

        stack.push(5); 

        stack.push(1);

        stack.push(5);

        Console.WriteLine(stack.top()); // 5

        Console.WriteLine(stack.popMax()); // 5

        Console.WriteLine(stack.top()); // 1

        Console.WriteLine(stack.peekMax()); // 5

        Console.WriteLine(stack.pop()); // 1

        Console.WriteLine(stack.top()); // 5

    }

}

​

public class MaxStack

{

    Stack<int> stack = new Stack<int>();

    Stack<int> maxStack = new Stack<int>();

    // push

    public void push(int value)

    {

        stack.Push(value);

        // push to maxStack if maxStack is empty or maxStack value is less than value

        if(maxStack.Count == 0 || maxStack.Peek() <= value) 

            maxStack.Push(value);               

    }

    public int top()

    {

        // returns top of the stack

        if(stack.Count > 0)

            return stack.Peek();

        else

            return -1;

    }

    public int popMax()

    {

        // return maximum value from MaxStack and also loops through stack to remove values until stack value = maxstack value

        if(maxStack.Count > 0)

        {

            int ret = maxStack.Peek();

            while(stack.Count > 0 && stack.Peek() != ret)

            {

                stack.Pop();

            }

            stack.Pop();

            return maxStack.Pop();

        }

        return -1;

    }

    public int peekMax()

    {

        return maxStack.Peek();

    }

    public int pop()

    {

        int ret = stack.Peek();

        if(ret == maxStack.Peek())

        {

            maxStack.Pop();

            return stack.Pop();

        }

        return stack.Pop();

    }

}

​

​

​

​

​

​