0077. Combinations

https://leetcode.com/problems/combinations

Description

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

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