Vertices of the Hull — Solution & Editorial

Back to the Problem

Andrew's monotone chain: sort points, build lower and upper hulls rejecting non-left turns (strict cross-product ≤ 0 removes collinear points). Count distinct hull vertices.

Complexity: O(N log N)

Watch out: Collinear points on the hull boundary: whether they count as vertices depends on the ≤ vs < in the turn test.