MA2

代码说明：

  构造一个空的双向栈，判断双向栈是否存在，出栈，入栈，判栈空，将栈置空，判栈满，取栈顶(Construct an empty two-way stack to determine whether there is a two-way stack, out of stack, stack, sub-stack empty, set the stack is empty, sub-stack is full, take the stack)