77. Combinations 
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Problem
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 <= 201 <= k <= n