Combinations That Hit the Target

You have a list of different positive whole numbers, candidates, and a target total. Write combination_sum(candidates, target) that returns every combination of candidates adding up to exactly target, where each candidate can be used as many times as you like.

Order inside a combination does not matter: [2, 3, 3] and [3, 2, 3] are the same combination and must appear once. Write each combination as a list in increasing order, and return the combinations sorted, in Python's ordering of lists.

Examples

combination_sum([2, 3, 5], 8)  # returns [[2, 2, 2, 2], [2, 3, 3], [3, 5]]
combination_sum([7, 3, 2], 7)  # returns [[2, 2, 3], [7]]
combination_sum([4, 6], 5)     # returns []

A total of 8 from 2, 3 and 5 can be made as four 2s, as a 2 and two 3s, or as a 3 and a 5.

Constraints

• 1 <= len(candidates) <= 30, and the candidates are all different.
• 1 <= candidates[i] <= 40
• 1 <= target <= 40