039 Combination Sum.py

'''
Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

The same repeated number may be chosen from C unlimited number of times.

Note:
All numbers (including target) will be positive integers.
Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
The solution set must not contain duplicate combinations.
For example, given candidate set 2,3,6,7 and target 7, 
A solution set is: 
[7] 
[2, 2, 3] 
'''

class Solution(object):
    def combinationSum(self, candidates, target):
        """
        :type candidates: List[int]
        :type target: int
        :rtype: List[List[int]]
        """
        if not candidates:
            return []
        candidates.sort()
        result = []
        self.combination(candidates, target, [], result)
        return result

    def combination(self, candidates, target, current, result):
        s = sum(current) if current else 0
        if s > target:
            return
        elif s == target:
            result.append(current)
            return
        else:
            for i, v in enumerate(candidates):
                self.combination(candidates[i:], target, current + [v], result)


if __name__ == "__main__":
    assert Solution().combinationSum([2, 3, 6, 7], 7) == [[2, 2, 3], [7]]