Problem - Combination Sum

Given an array of distinct integers and a target number, return all unique combinations where the chosen numbers sum to the target. You may use the same number multiple times.

For example, if candidates $= [2, 3, 6, 7]$ and target $= 7$, the answer is $[[2, 2, 3], [7]]$. Notice you can use $2$ twice.

This combines the combination-building pattern you just learned with a sum constraint. The twist: numbers can repeat.