LeetCode: Subsets [078]

class Solution {
public:
    
    void dfs(vector<vector<int> >&result, vector<int>combination, vector<int>&candidates, int kth, int k, int index2add){
        // 当前正在确定组合中的第kth个数，将把候选集candidates中index2add索引位的值作为第kth个数加到组合中
        combination.push_back(candidates[index2add-1]);
        if(kth==k){
            result.push_back(combination);
        }
        else{
            for(int i=index2add+1; i+(k-kth-1)<=candidates.size(); i++){
                dfs(result, combination, candidates, kth+1, k, i);
            }
        }
    }
    
    vector<vector<int> > getKsubset(vector<int> &S, int k){
        // 获得k元子集合
        vector<vector<int> >result;
        vector<int> combination;
        for(int i=1; i+(k-1)<=S.size(); i++){
            dfs(result, combination, S, 1, k, i);
        }
        return result;
    }

    vector<vector<int> > subsets(vector<int> &S) {
        vector<vector<int> >result;
        //一定有一个空集
        vector<int> nonVector;
        result.push_back(nonVector);
        int size=S.size();
        if(size==0)return result;
        //排序
        sort(S.begin(), S.end());
        for(int k=1; k<=size; k++){
            //依次找k元组合
            vector<vector<int> >ksubsets = getKsubset(S, k);
            result.insert(result.end(), ksubsets.begin(), ksubsets.end());
        }
        return result;
    }
};