Evaluate Reverse Polish Notation

Write `eval_rpn(tokens)` evaluating a list of tokens in postfix notation. Tokens are integers as strings, or one of `+ - * /`. Division truncates toward zero. eval_rpn(["2", "1", "+", "3", "*"]) -> 9 eval_rpn(["4", "13", "5", "/", "+"]) -> 6 Two traps, both about order. The second value popped is the left operand, so `a - b` and `a / b` break if you swap them. And Python's `//` floors rather than truncating, so `-7 // 2` is `-4` where this problem wants `-3`.