Need to find an algorithm that given 2 sequences of n numbers each, say S1 and S2, and 1 number, say x, it would tell if x is in the sum result of S1 and S2. The hashing function, say h, must be picked at random from a universal set of hashing functions, say H. The algorithm should not take more than O(n) time.
Assume H is a universal hashing family, from which h(z) is picked at random. Use h on every element, say s1 in S1 and s2 in S2 to compute h(s1 + s2) and place result in table of size n, A[1,....,n].
Check if A[h(z)] is occupied. If it is then z is in sum of S1 and S2.