Chef and Isomorphic Array

Today, Chef came up with the following problem: Consider two arrays a and b with the same size. Denote the multisets of all elements of arrays a and b by MA and MB respectively. The arrays a, b are isomorphic if it's possible to find a bijective function f: Z → Z which satisfies the following rules: - Let's construct a new multiset MF. For each element x of MA, we should insert f(x) into MF.