RPN_52.pas

program RPN;
uses Contnrs;
var
  s: String; //Input string
  stack: TStack; //Working RPN stack
  a, b, r: LongInt; //Two operands and result
  pa, pb, pr: ^LongInt;
  op: Char;
  buf: String; //Holds a single operand or operator
  i, l, head, tail: Integer; // Input string parser pointers

begin
  ReadLn(s);
  s := s + ' ';
  i := 0; //infinite loop detection
(*  New(pa);
  New(pb);
  New(pr); *)
  a:=-1; pa := @a;
  b:=-1; pb := @b;
  r:=-1; pr := @r;
  stack := TStack.Create;

  repeat
    head := 1;
    tail := Pos(' ', s);
    buf := Copy(s, head, tail-head);

    op := buf[1];
    case op of // Pascal case does not fall through!
      '+':
      begin
        pa := stack.Pop; a:=pa^;
        pb := stack.Pop; b:=pb^;
        r := a + b;
        stack.Push(pr);
      end;
      '-':
      begin
        pa := stack.Pop; a:=pa^;
        pb := stack.Pop; b:=pb^;
        r := a - b;
        stack.Push(pr);
      end;
      '*':
      begin
        pa := stack.Pop; a:=pa^;
        pb := stack.Pop; b:=pb^;
        r := a * b;
        stack.Push(pr);
      end;
      '0'..'9':
      begin
        Val(buf,r); pr:=@r;
        stack.Push(pr);
      end;
    end; //case
//    WriteLn(buf); //debug
    Delete(s, head, tail-head+1); // delete the space as well

    Inc(i); //infinite loop detection
    l := Length(s);
  until (l=0) or (i>32768);
//  WriteLn(s); // is empty

  pr := stack.Pop;
  WriteLn(pr^);

  (*Dispose(pa);
  Dispose(pb);
  Dispose(pr);*)
  stack.Destroy;
//  ReadLn;
end.