Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
You may return the answer in any order.
Input: n = 4, k = 2
Output:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]
Input: n = 1, k = 1
Output: [[1]]