原题网址：https://www.lintcode.com/problem/k-sum-ii/description
Given n unique integers, number k (1<=k<=n) and target.
Find all possible k integers where their sum is target.
给出[1,2,3,4]，k=2， target=5，返回 [[1,4],[2,3]]
原文：https://www.cnblogs.com/Tang-tangt/p/9319849.html