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

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

         public T GetMax()
         {
             if (maxStack.Count > )
                 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()) > )
             {
                 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 "":
                         Console.Write("请输入入队值(int):");
                         var value = Console.ReadLine();
                         int v = ;
                         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 "":
                         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 "":
                         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;
         }
     }
 }