The problem
Evaluate a postfix expression given as tokens (e.g. ["2","1","+","3","*"] = 9). Division truncates toward zero.
Stuck? Reveal hints one at a time
How to approach it
- 1Scan tokens. Numbers → push.
- 2Operator → pop b then a, compute a op b, push the result.
- 3One value remains at the end — the answer.
Key insight
RPN is what compilers reduce infix to precisely because a single stack evaluates it in one pass with zero grammar.
The solution
Watch out for
- Python’s // floors toward −∞; the problem wants truncation toward zero — use int(a/b).
- Negative numbers arrive as tokens like "-3": test for operators, not for "starts with -".