문제 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. 예시 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]] 제약 조건 Constraints: 1