Output the size of its longest contiguous block made up of the same character.
Although the request is brief, it may implicitly ask how you would use a character-frequency map.
Notes
Boundary conditions: an empty input produces 0, while a one-character input produces 1.
Preparation
Review LC 340, "Longest Substring with At Most K Distinct Characters," as the expected follow-up: its prompt looks similar, but its solution strategy is substantially different.