//Stack STL

//在STL中，栈是以别的容器作为底部结构，再将

//接口改变，使之符合栈的特性

//一共5个常用操作函数

//构造析构

stack<Elem>c;     //build a empty stack

stack<Elem>c1(c2);   //copy a stack

//5 functions

c.top();      //return the element  at the top of the stack

c.push(elem);     //push element at the top

c.pop();     //pop element at the top

c.empty();

c.size();    //return the size of the stack

//Stack is just a encapsulation of other container.

//It just supply its own interfaces.

//Elements are pushed/popped from the "back" of the specific container, which is //known as the top of the stack.