数字组合

vector<vector<int> > combinationSum(vector<int> &candidates, int target) {  
        vector<vector<int>> result;
        if (&candidates == NULL)
        {
            return result;
        }

        vector<int> path;
        sort(candidates.begin(), candidates.end());
        helper(candidates, target, path, 0, result);

        return result;
    }

    void helper(vector<int> &candidates, int target, vector<int> path, int pos, vector<vector<int>> &result)
    {
        if (target == 0)  // 找到一组满足条件的数字组合
        {
            result.push_back(path);
            return;
        }
        int prev = -1;
        for (int i = pos; i < candidates.size(); i++) // 从pos开始是因为保证不取到重复的情况
        {
            if (candidates[i] > target)  // 找不到满足条件的解
            {
                break;
            }

            if (prev != -1 && prev == candidates[i]) // != -1表示从数组【3,6,7】开始，== candidates[i]表示candidates[i]与candidates[i-1]重复了，需要抛弃它，因为candidates[i-1]是不满足条件
            {
                continue;
            }

            path.push_back(candidates[i]);
            helper(candidates, target - candidates[i], path, i ,result);
            path.pop_back();

            prev = candidates[i];  // 前一个加入path的数为prev


        }
    }