《剑指offer》面试题7—用两个栈实现队列

题目：给出队列声明，要求实现AppendTail和DeleteHead函数。

template <typename T>
class CQueue
{
public:
void AppendTail(const T& element);
T DeleteHead();
private:
stack<T> stack1;
stack<T> stack2;
};

思路：要用两个先进后出的栈实现先进先出的队列。压栈总是压入stack1，要获得队列头时：如果stack2为空，则把stack1依次弹出栈并压入stack2，取stack2栈顶元素；如果stack2不为空，则直接取stack2栈顶元素。

 #include <iostream>
 #include <stack>
 using namespace std;

 template <typename T>
 class CQueue
 {
 public:
     void AppendTail(const T& element);
     T DeleteHead();
 private:
     stack<T> stack1;
     stack<T> stack2;
 };

 template<typename T> void CQueue<T>::AppendTail(const T& element)
 {
     stack1.push(element);
 }

 template<typename T> T CQueue<T>::DeleteHead()
 {
     T temp;
     if(!stack2.empty())
     {
         temp = stack2.top();
         stack2.pop();
     }
     else
     {
         if(stack1.empty())
         {
             cout<<"No element left!"<<endl;
             //return;
         }
         else
         {
             while(!stack1.empty())
             {
                 temp = stack1.top();
                 stack1.pop();
                 stack2.push(temp);
             }
             temp = stack2.top();
             stack2.pop();
         }
     }
     return temp;
 }

 int main()
 {
     CQueue<int> my_queue;
     int n,val;
     cout<<"1 for Append"<<endl<<"2 for DeleteHead"<<endl;
     while(scanf("%d",&n)!=)
     {
         if(n == )
         {
             scanf("%d",&val);
             my_queue.AppendTail(val);
         }
         else if(n == )
         {
             val = my_queue.DeleteHead();
             cout<<"The head of Queue:"<<val<<endl;
         }
         else
             cout<<"Illegal n!"<<endl;
     }
     return ;
 }