O(1)取Queue中的最大值

实现原理：

1、利用Stack的先进后出的特性，实现一个MaxStack，MaxStack中用一个Stack记录当前的值，一个Stack记录当前的最大值。

2、用2个MaxStack实现MaxQueue，其中一个MaxStack记录出栈记录，一个MaxStack记录入栈记录，取2个MaxStack中的最大值作为当前MaxQueue的最大值。

using System;
using System.Data;
using System.Collections;
using System.Collections.Generic;

namespace HelloWorld
{
    public class MaxStack<T>
    {
        private Stack<T> data;
        private Stack<T> maxStack;

        public MaxStack()
        {
            data = new Stack<T>();
            maxStack = new Stack<T>();
        }

        public void Push(T element)
        {
            if (data.Count == 0)
            {
                data.Push(element);
                maxStack.Push(element);
            }
            else
            {
                var current = maxStack.Peek();
                var compare = Comparer<T>.Default;
                if (compare.Compare(current, element) < 0)
                {
                    maxStack.Push(element);
                }
                data.Push(element);
            }
        }

        public T Pop()
        {
            if (this.IsEmpty)
                throw new Exception("Queue is empty.");
            else
            {
                var current = data.Pop();
                var compare = Comparer<T>.Default;
                if (compare.Compare(current, maxStack.Peek()) == 0)
                {
                    maxStack.Pop();
                }
                return current;
            }
        }

        public bool IsEmpty
        {
            get
            {
                return data.Count == 0;
            }
        }

        public T GetMax()
        {
            if (maxStack.Count > 0)
                return maxStack.Peek();
            else
                return default(T);
        }
    }
}

using System;
using System.Data;
using System.Collections;
using System.Collections.Generic;

namespace HelloWorld
{
    public class MaxQueue<T>
    {
        private MaxStack<T> popStack;
        private MaxStack<T> pushStack;
        public MaxQueue()
        {
            popStack = new MaxStack<T>();
            pushStack = new MaxStack<T>();
        }

        public void Enqueue(T element)
        {
            pushStack.Push(element);
        }

        public T Dequeue()
        {
            if (popStack.IsEmpty && pushStack.IsEmpty)
                throw new Exception("Queue is empty.");
            if (popStack.IsEmpty)
            {
                while (!pushStack.IsEmpty)
                {
                    var element = pushStack.Pop();
                    popStack.Push(element);
                }
            }
            return popStack.Pop();
        }

        public T GetMax()
        {
            Comparer comparer = Comparer.Default;
            if (comparer.Compare(popStack.GetMax(), pushStack.GetMax()) > 0)
            {
                return popStack.GetMax();
            }
            else
            {
                return pushStack.GetMax();
            }
        }
    }
}

using System;
using System.Text;

namespace HelloWorld
{
    class Program
    {
        static void Main(string[] args)
        {
            MaxQueueTest();
        }

        static void MaxQueueTest()
        {
            MaxQueue<int> queue = new MaxQueue<int>();
            SELECT:
                var selected = ShowMenu();
                switch (selected)
                {
                    case "1":
                        Console.Write("请输入入队值(int):");
                        var value = Console.ReadLine();
                        int v = 0;
                        if (int.TryParse(value, out v))
                        {
                            queue.Enqueue(v);
                            Console.WriteLine("输入入队值:{0}后，,当前最大值为:{1}", value,queue.GetMax());
                        }
                        goto SELECT;
                    case "2":
                        try
                        {
                            var element  = queue.Dequeue();
                            Console.WriteLine("出队值:{0},当前最大值为:{1}",element,queue.GetMax());
                        }
                        catch(Exception ex)
                        {
                            Console.WriteLine(ex.Message);
                        }
                        goto SELECT;
                    case "3":
                        break;
                    default:
                        Console.WriteLine("输入有误，请重新选择。");
                        goto SELECT;
                }
        }

        static string ShowMenu()
        {
            MaxQueue<int> queue = new MaxQueue<int>();
            Console.WriteLine("*****************************************");
            Console.WriteLine("1、入队");
            Console.WriteLine("2、出队");
            Console.WriteLine("3、退出");
            Console.WriteLine("*****************************************");
            Console.Write("请选择:");
            var selected = Console.ReadLine();
            return selected;
        }
    }
}