Longest Substring of All Same Letter

Given a string, return the length of the longest contiguous substring consisting of a single repeated character. A simple linear-scan warm-up in VP-level phone screens. Requirements Input: a string. Return: the length of the longest run of a single repeated character. The prompt is terse and may carry an implicit follow-up about handling a per-character count map. Notes Single linear pass with a runStart / runLength pair, or equivalently with two pointers left / right. O(n) time, O(1) space. Likely follow-up: "now find the longest substring with at most K distinct characters" (LC 340) — different algorithm, sliding window with a count map. Be ready for the pivot. Edge cases: empty string returns 0; single character returns 1. Preparation Drill once cleanly with the two-pointer scan. Read LC 340 "Longest Substring with At Most K Distinct Characters" as the natural follow-up — same prompt structure, fundamentally different solution.