LeetCode subset dfs

class Solution {
public:
    vector<vector<int> > subsets(vector<int> &S) {
        vector<vector<int>> res;
        vector<int> tmp;
        sort(S.begin(),S.end());
        dfs(res,tmp,0,S);
        sort(res.begin(),res.end(),cmp);
        return res;
    }
    
    static bool cmp(vector<int> a,vector<int> b){
        if(a.size()==b.size()){
            for(int i=0;i<a.size();i++)
                if(a[i]!=b[i])
                    return a[i]<b[i];
        }
        return a.size()<b.size();
    }
    
    void dfs(vector<vector<int>> &res,vector<int> &tmp,int pos,vector<int> a){
        res.push_back(tmp);
        for(int i=pos;i<a.size();i++){
            tmp.push_back(a[i]);
            dfs(res,tmp,i+1,a);
            tmp.pop_back();
        }
    }
};