Make the Reverse Polish Notation calculator work.

____PART ONE_____

  We went through the implementation of an RPN calculator in class.
  Make it into a complete C++ program and make sure it works properly.

  Make sure you use a vector of ints or doubles in your implementation.

  Here's a small sample of the sort of thing that should work:

  1 2 3 4 5 + + + + end
  result is 15

  123 456 + end
  result is 579

  1 2 + 3 + 4 + 5 + end
  result is 15

  1 2 * 3 4 * + 5 + end
  result is 19

  1 2 + 3 4 + * 5 * end 
  result is 105

____PART TWO_____

  Make variables work

  If the input is
    2 3 4 + * = a end
  that means work out 2*(3+4) and remember that as the value of a.
  = needs to be handled specially, don't treat it as a normal operator

  If the input is
    2 a * end
  then the result should be whatever 2 * a is

  You should define a struct/object to represent the calculator's
  memory. As well as a constructor, it should have these three methods:

    void set(string variablename, double value);
    bool isdefined(string variablename);
    double getvalue(string variablename);

  There should be no limit on the number of variables that your memory
  can store.

____INSTRUCTIONS_____

Make the two parts be separate programs, so that if you do well on part
1 but make a mess of part 2, you can still get full marks for part 1.