Question
A username analyzer reports each handle's 'busiest letter'. Given a handle containing at least one ASCII letter (plus possibly digits, underscores, or mixed case), return the letter that occurs most often, comparing case-insensitively and ignoring non-letters. Return it as a lowercase one-character string; break ties by choosing the alphabetically smallest letter. For example, 'Mississippi' returns 'i' (i and s both appear four times).
busiest_letter(handle: str) → str["Mississippi"]out"i"State 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.