You are given two integer arrays, list1 and list2, sorted in non-decreasing order, and an integer, k.

A pair $(u, \space v)$ is defined as one element $u$ chosen from list1 and one element $v$ chosen from list2.

Your task is to return the k pairs $(u1, \space v1), (u2, \space v2), ..., (uk, \space vk)$ whose sums $u1 + v1, u2 + v2, ..., uk + vk$ are the smallest among all possible such pairs.