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(LeetCode 77)
Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].
Example: n=4, k=2 returns [[1,2], [1,3], [1,4], [2,3], [2,4], [3,4]]. There are C(4,2) = 6 combinations.
Read the full problem statement, then come back for the walkthrough.
