77\. Combinations

Medium

Given two integers `n` and `k`, return _all possible combinations of_ `k` _numbers out of the range_ `[1, n]`.

You may return the answer in **any order**.

**Example 1:**

**Input:** n = 4, k = 2

**Output:** [ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ] 

**Example 2:**

**Input:** n = 1, k = 1

**Output:** [[1]] 

**Constraints:**

*   `1 <= n <= 20`
*   `1 <= k <= n`