def combinationSum2(candidates, target):
    res = []
    candidates.sort()
    
    def dfs(target, index, path):
        if target < 0:
            return  # backtracking
        if target == 0:
            res.append(path)
            return 
        for i in range(index, len(candidates)):
            dfs(target-candidates[i], i, path+[candidates[i]])
            
    dfs(target, 0, [])
    return res