用两个栈实现一个队列

思路：

      主要抓住栈的“先进后出”和队列的“先进先出”。

      s1是入栈的，s2是出栈的。

1. 入队列，直接压到s1是就行了。
2. 出队列，先把s1中的元素全部出栈压入到s2中，弹出s2中的栈顶元素；再把s2的所有元素全部压回s1中。

代码实现：

#include <iostream>

using namespace std;

template <class T>

class Stack

{

public :

     Stack():_stack(NULL),_capacity(0),_top(0)

     {

     }

     Stack( int capacity):_stack(NULL),_top(0)

     {

          assert(capacity > 0); //确保指定的栈的初始容量大于零

          _stack = new T[capacity];

     }

     ~Stack()

     {

           delete[] _stack;

          _top = 0;

          _capacity = 0;

     }

      void Push(T elem)

     {

           if(_top == _capacity)// 若当前栈已满，则进行扩容

          {

              T* temp = new T[_capacity*2+1];

              _capacity = _capacity*2+1;

              memcpy(temp,_stack, sizeof(T)*_top);

               delete[] _stack;// 将当前的栈空间释放

              _stack = temp;

          }

          _stack[_top++] = elem;

     }

      void Pop()

     {

           if(Empty())

          {

              

               return;

          }

          

          _stack[--_top] = NULL;

     }

      void Pop(T& elem)

     {

          elem = _stack[--_top];

          stack[_top] = NULL;

     }

 

      const T& Top()

     {

           return _stack[_top - 1];

     }

      bool Empty()

     {

           return _top == T();

     }

private :

     T* _stack; //定义指向栈数组的指针

      int _capacity;// 栈的容量

      int _top;// 栈顶的下一个位置处下标

};

template <class T>

class Queue

{

public :

     Queue()

     {

     }

     ~Queue()

     {}

      bool Empty()

     {

           return(s1.Empty()&&s2.Empty());

     }

      void Push(T elem)

     {

      if(s1.Empty()&&!s2.Empty())

          {

               while(!s2.Empty())

              {

                   s1.Push(s2.Top());

                   s2.Pop();

              }

              s1.Push(elem);

           }

           else 

              s1.Push(elem);

     }

      void Pop()

     {

           if(Empty())

          {

              cout<< "Queue is not Pop" <<endl;

              exit(-1);

          }

           if(s1.Empty()&&!s2.Empty())

          {

              s2.Pop();

          }

           if(!s1.Empty()&&s2.Empty())

          {

               while(!s1.Empty())

              {

                   s2.Push(s1.Top());

                   s1.Pop();

              }

              s2.Pop();

          }  

     }

      const T& Top()

     {

           if(Empty())

          {

              cout<< "Queue is NULL!!!" <<endl;

              exit(-1);

          }

           while(!s1.Empty())

          {

              s2.Push(s1.Top());

              s1.Pop();

          }

          T tmp =  s2.Top();

           while(!s2.Empty())

          {

              s1.Push(s2.Top());

              s2.Pop();

          }

           return tmp;

     }

      void Print()

     {

           while(!s1.Empty())

          {

              s2.Push(s1.Top());

              s1.Pop();

          }

           while(!s2.Empty())

          {

              cout<<s2.Top()<< " ";

              s2.Pop();

          }

          cout<<endl;

     

     }

private :

     Stack<T> s1;

     Stack<T> s2;

};

int main()

{

     Queue< int> q1;

     for(int i=0;i<100;++i)
     {
        q1.Push(i);
     }

     q1.Pop;

     

     q1.Print();

     system( "pause");

      return 0;

}