Find the Celebrity

Given n people labeled 0 to n-1 and an API knows(a, b) that returns true if person a knows person b, find the celebrity — someone who is known by everyone else but knows no one — or return -1 if none exists. The core challenge is to identify the candidate with minimal knows() calls (typically via pairwise elimination) and then verify incoming/outgoing relationships, achieving O(n) time and O(1) space.