Problem - Combinations

Given two integers $n$ and $k$, return all possible combinations of $k$ numbers chosen from the range $[1, n]$. For example, if $n = 4$ and $k = 2$, you return $[[1,2], [1,3], [1,4], [2,3], [2,4], [3,4]]$.

This is the classic combinations problem. You learned $C(n, k) = \frac{n!}{k!(n-k)!}$ counts how many combinations exist. Now you will generate them all.

The key constraint: order does not matter. $[1, 2]$ and $[2, 1]$ are the same combination. You need to avoid duplicates.