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