How do you find the longest valid parentheses substring?
PICTURE THIS: STACK VS QUEUE
Simple meaning
Stack unmatched '(' indices plus a base of the last unmatched ')'
WHY — Strings instead of guessing?
Why interviewers care about Strings:
question about Strings.
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 step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Stack unmatched '(' indices
plus a base of the last unmatched ')'
- 2on a match the
length is i minus the new top.
- 3DP is an alternative
where dp[i] extends a previous valid block
- 4both are O(n) time
and O(n) space.
- 5A two-pass left/right counter
trick can use O(1) extra space.
- 6Close
When you pick this over the alternative.
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
Stack unmatched '(' indices plus a base of the last unmatched ')' on a match the length is i minus the new top.