Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].

You may return the answer in any order.

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. 

Constraints:

1 <= n <= 20

1 <= k <= n

class Solution:

def combine(self, n: int, k: int) -> List[List[int]]:

Implementation in python3