How do you evaluate a postfix Reverse Polish expression?
PICTURE THIS: A SENTENCE BECOMES TOKENS
The model does not read letters like humans. It reads these pieces, then predicts the next one.
Simple meaning
Scan tokens: push numbers, and for an operator pop two operands, apply it, and push the result.
WHY — Stack instead of guessing?
Why interviewers care about Stack:
question about Stack.
trade-offs, and what you would actually do on a DSA project - not buzzwords.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens with tokens?
Before the model can read a sentence, it goes through these steps:
- 1Scan tokens: push numbers,
and for an operator pop two operands, apply it, and push the result.
- 2A well-formed expression leaves
one value on the stack.
- 3Time is O(n) and
space is O(n) for the stack.
- 4Prefix is similar but
often scanned right to left.
- 5Next token
The model scores what should come next.
- 6Decode
IDs turn back into readable text.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Scan tokens: push numbers, and for an operator pop two operands, apply it, and push the result. A well-formed expression leaves one value on the stack.