《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)!=0)
    {
        if(n == 1)
        {
            scanf("%d",&val);
            my_queue.AppendTail(val);
        }
        else if(n == 2)
        {
            val = my_queue.DeleteHead();
            cout<<"The head of Queue:"<<val<<endl;
        }
        else
            cout<<"Illegal n!"<<endl;
    }
    return 0;
}