- LeetCode 208 – Implement Trie (Prefix Tree)
A build-the-structure question where the interview is really two things: why a trie beats a hash set for prefix queries, and the one boolean separating a word from a prefix. Every operation is independent of how many words are stored.
- LeetCode 207 – Course Schedule
Cycle detection in a directed graph wearing a scheduling problem's clothes. Kahn's algorithm never looks for the cycle — it notices what is left over. And the DFS version needs THREE node states, because already-finished and currently-above-me are different facts.
- LeetCode 206 – Reverse Linked List
The most-asked list question there is, and it is asked because four assignments in the wrong order lose the rest of the list. Save the successor before overwriting the link, return previous rather than current — and it is a building block for half the harder list problems.
- 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.