Question
Return the length of the longest substring that appears contiguously in BOTH strings a and b (the longest common substring, not subsequence). For example "abcdxyz" and "xyzabcd" share "abcd" of length 4. Either string may be empty (answer 0). Strings can be up to ~1000 characters; an O(len(a)*len(b)) DP is acceptable, though suffix automata/arrays give better asymptotics.
longest_common_substring_len(a: str, b: str) → int["AcornforAcorn","AcornQuiz"]out5State your approach and its time/space complexity out loud before you optimize. Handle the edge cases (empty input, duplicates, overflow), and say why you chose this over the brute force. Green tests are the floor, not the grade.
Vibe coding: describe the solution in plain language (or narrate it) and the coach grades your approach. Generating runnable code from your description is coming next.