My first instinct was sliding window but I kept conflating 'at most n distinct' with 'exactly n distinct' and that cost me probably five minutes of confused code.
Clarify the problem, including edge cases and constraints, then propose a sliding window approach with a hash map to track character frequencies. Explain how to maintain exactly n distinct characters by expanding and shrinking the window, and analyze time and space complexity before coding.
Pro tip: During implementation, use a while loop to shrink the window when distinct count exceeds n, and update the minimum length only when distinct count equals n. This avoids unnecessary checks and keeps the code clean.
Ask about input constraints, character set, and expected behavior for cases like n=0, n greater than distinct characters in string, or empty string. Confirm that substring must be contiguous.
Propose a sliding window with two pointers and a frequency map. Explain that each character is processed at most twice, giving O(m) time where m is string length, and O(k) space where k is distinct characters.
Describe expanding right pointer, updating frequency map, and when distinct count exceeds n, shrink from left until distinct count <= n. Track minimum length when distinct count equals n.
Write clean code with meaningful variable names, handling edge cases upfront. Use a hash map or array for frequencies, and maintain distinct count efficiently.
Run through provided examples and edge cases. Discuss alternative approaches (e.g., brute force) and why sliding window is optimal. Mention potential optimizations if character set is small.
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