Only the top element of the stack has a chance to be accessed by the outside world. The stack does not provide traversal functions, nor does it provide iterators.
stack constructor
stack<T> stkT; // stack采用模板类实现，默认构造
stack（const stack &stk）； // 拷贝构造函数
stack assignment operation
stack& operator=(const stack &stk); // 重载=
stack data access
push(elem); // 向栈顶添加元素
pop(); // 从栈顶移除第一个元素
top(); // 返回栈顶元素
Stack size operation
empty(); // 判断栈是否为空
size(); // 返回栈的大小