Evaluate Reverse Polish Notation
Signals to notice
Brute force first
No better approach exists — stack is the natural solution. That instinct is useful because it follows the prompt literally, but it usually keeps revisiting work the problem is begging you to organize.
The key insight
Push numbers onto stack; when you see an operator, pop two operands, compute, push result. The goal is not to be clever for its own sake, but to remember the one relationship that keeps the solution grounded as you move forward.
What must stay true
The stack always contains operands waiting for their operator. If that remains true after every update, the rest of the reasoning has a stable place to stand.
Easy way to go wrong
Operand order matters for subtraction and division — second popped is left operand. When the code becomes mechanical before the idea is clear, small edge cases start breaking the whole story.