Currently, n is defined in your constructor. The problem is, when the constructor ends, n goes away (we call that, "going out of scope").
So, you have a few ways to solve this problem.
First, you can declare n in your push method. Keep in mind that this n might (and, in this case, would) reference a different object than the n declared in your constructor.
Second, you can declare n as a member variable, just as top, aux, and fin are.
Lastly, you can get rid on n altogether. The object referenced by n in your constructor is never used outside your constructor. Judging from how you are trying to use "n," it seems likely to me that you want to use the keyword "this" instead. "this" refers to the current object, which seems to be what you are trying. (Note that you are using "this" implicitly in the line that says "top = temp;" you could also say "this.top = temp;" with no problem.")
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