//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 stackstack<Elem>c1(c2);   //copy a stack//5 functionsc.top();      //return the element  at the top of the stackc.push(elem);     //push element at the topc.pop();     //pop element at the topc.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.