如果我们有两个相同类型的栈，我们为他们各自开辟了数组空间，极有可能第一个栈已经满了，再进栈就溢出了，而另一个栈还有很多存储空间空闲。这时，我们完全可以用一个数组两存储两个栈。(f we have two identical types of stack, we have opened up the array space for them. It is very likely that the first stack is full, then the stack will overflow, and the other stack will have many storage space. At this point, we can store two stacks with an array two completely.)