strings

A 5-minute warm-up that quietly tests the most useful idea in practical coding: choosing a canonical key. The sorted-string vs character-count tradeoff is exactly what interviewers want to hear you reason about.

cd('/foo/bar', 'baz') is easy. cd('/foo/../', './baz') is where the OpenAI screen actually starts, and the symlink follow-up is where it ends. The stack solution plus the loop-guard most candidates forget.

No JSON libraries allowed, and the test data contains commas, newlines, and your escape character itself. Length-prefix encoding solves it in fifteen lines, and knowing why it beats escaping is the actual interview.

A DP classic that screens for one thing: can you define the right subproblem and fill the table without fumbling the indices. The strong version adds the O(min(m,n)) space trick and reconstructs the actual edits when the follow-up lands.

A sliding window with a last-seen index map, O(n) in one pass. The whole interview turns on one subtlety: when you hit a repeat, the window's left edge must jump forward and never slide backward, which is exactly the bug most candidates ship.

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.

The signature coding problem at a legal-AI company, and it is interval merging wearing a product costume. The candidates who struggle are the ones who start matching before deciding what a match even is.