- LeetCode 205 – Isomorphic Strings
A one-map solution that is wrong and a two-map solution that is right, separated by a single word in the problem statement. The pair badc / baba passes every consistency check and is still not isomorphic, because b and d both map to b.
- LeetCode 204 – Count Primes
A problem about knowing an algorithm rather than deriving one. What is actually tested is the two optimisations that make the sieve fast — stop at sqrt(n), start the inner loop at p*p — and whether you can say why both follow from one fact.
- MongoDB – Text Search
Text indexes and field weights, the one-per-collection limit, and the point where you should stop and reach for a real search engine.
- LeetCode 203 – Remove Linked List Elements
The problem that proves the dummy-head rule problem 83 stated. Here the head CAN be removed, repeatedly, so the dummy stops being a preference and becomes what makes the code short. And you must return dummy.next, never head.
- LeetCode 202 – Happy Number
Linked List Cycle with the list replaced by a function. No nodes, no next pointer, and Floyd's algorithm works anyway — which is the point: cycle detection needs a successor function, not a data structure. Plus why the sequence must terminate at all.