https://leetcode.com/problems/combinations/
77. Combinations
给你一个数字 n 和 k ，返回 Cnk 的组合。
Example 1:
Input: n = 4, k = 2
Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
Explanation: There are 4 choose 2 = 6 total combinations.
Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to
be the same combination.
Example 2:
Input: n = 1, k = 1
Output: [[1]]
Explanation: There is 1 choose 1 = 1 total combination.
思路：
1.回溯法经典题，dfs穷举所有可能的结果，并做一些剪枝后返回。
Java Code：