[LeetCode] 77. Combinations

題目：
Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

思路：
用遞迴的方式, 從後面排回來
12
13
   -> 23
14
   -> 24
   -> 34

n = 4, k = 3
123
124
   -> 134
           -> 234

https://gist.github.com/richardneko/6870d498e3916733728917b0d9c8ba7f