mediumMetaAmazonMicrosoft

Valid parentheses, then generate all valid combinations

A two-part screen that looks like a warm-up and isn't. Part one is the classic stack validator with three bracket types; part two flips to generating every valid string of n pairs, and the Catalan count is the detail that catches people off guard.

Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.

20 answers per topic instead of 10, plus saved progress and bookmarks · no cardor unlock all 559 remaining answers · ₹2,000 / $25

Every answer, concept and course, every paper breakdown, all published video lessons, hands-on FDE Lab missions, Premium PDF guides and companion files, the full practice-test bank and work-sample downloads. Referral Premium excludes guide PDFs and their companion files.

6 months · One payment · No auto-renewal

Study alongside free video lessons.

Keep practising

Next in this trackGiven a biased coin, produce a fair coin flipPremium Next in this trackA streaming chat UI in React drops tokens, shows garbled characters, and lags on long replies. Find the three bugs and fix them.Premium Next in this trackSimulate an infection spreading across a grid over timePremium

FEDITOR'S NOTE

The pivot from validation to generation is deliberate: the interviewer wants to see you switch from a linear scan to a backtracking search without restarting your thinking. The reliable filter is whether you prune with the open/close counters instead of generating all 2^(2n) strings and validating each.

DISCUSSION · 0

No comments yet — be the first to share your approach.