Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].
n
k
[1, n]
You may return the answer in any order.
1 <= n <= 20
1 <= k <= n
n = 4, k = 2
[[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
n = 1, k = 1
[[1]]