Combination Sum

class Solution:
    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:

        def backtrack(remain: int, combs: List[int], index: int) -> None:

            if remain == 0:
                # this ensures that each solution stored in results
                # is independent of future modifications to combs.
                results.append(list(combs))

            if remain < 0:
                return

            # recursive: append the number
            for i in range(index, len(candidates)):
                combs.append(candidates[i])
                backtrack(remain - candidates[i], combs, i)
                combs.pop()

        results = []
        backtrack(target, [], 0)
        return results