DSA: Find the longest substring with at most 2 distinct characters

Start by clarifying the exact contract for Longest Substring Two Distinct: input representation, mutation rules, duplicate behavior, and what should happen on invalid or empty input. The core idea is sliding-window invariant with a frequency map. Work a tiny example aloud and name the invariant after each step, because that is the proof that the approach stays correct as the input grows.

A dependable implementation plan is to expand right; increment frequency; shrink left while distinct count exceeds two; delete zero-count keys; update best valid length. Resetting the full window loses valid suffixes. Keep the code aligned with that invariant instead of adding special cases that silently change the contract. Test the smallest input, a normal case, duplicates or ties where relevant, an adversarial boundary, and a case that exercises the failure path. The expected complexity is O(n) time and O(k) space. Explain both time and auxiliary memory, including recursion or helper structures; do not hide sorting, copying, or repeated scans. If the title omits critical constraints, say what you need to know before committing to a more specialized solution.