Find K Pairs with Smallest Sum (Leetcode 373/Lintcode 1274)[Solution Illustrated]

Problem — Find K Pairs with Smallest Sums

You are given two integer arrays nums1 and nums2 sorted in ascending order and an integer k. Define a pair (u,v) which consists of one element from the first array and one element from the second array.
Find the k pairs (u1,v1),(u2,v2) …(uk,vk) with the smallest sums.
Return results need to be orderly.