(a) When is a collection of hash function universal? Provide a formal definition.

(b) H is a set of two hash functions h1 and h2, defined below, from the universe U = {1, 2, 3, 4} to {0, 1}.

U	1	2	3	4
h1	0	1	1	0
h2	0	0	1	1

Is H universal? Explain your answer.