Find a special person using knows(a,b)

Problem

You are given n people labeled 0 to n-1 at a party. You can call an API:

• knows(a, b) -> bool: returns true if person a knows person b, otherwise false.

A special person is defined as someone who:

1. Is known by everyone else (for all i != x, knows(i, x) == true), and
2. Knows nobody else (for all i != x, knows(x, i) == false).

Return the label of the special person if one exists, otherwise return -1.

Constraints

• 1 <= n <= 10^4
• You should minimize the number of API calls; aim for O(n) calls.

Notes

• knows(a, a) is undefined and should not be relied upon.