集合的子集

class Permutation {
public:
    vector<string> getPermutation(string A) {
        vector<string> res;
        if(A.size() == 0)
            return res;
        
        int cur = 0;        
        permutation(A,res,cur);
        sort(res.begin(),res.end(),greater<string>());
        
        return res;
    }
    
    void permutation(string A,vector<string> &res,int cur){
        int len = A.size();
        
        if(cur == len - 1){
            res.push_back(A);
            return;
        }
        
        for(int i = cur;i < len;i++){
            swap(A[i],A[cur]);
            permutation(A,res,cur+1);
            swap(A[i],A[cur]);
        }
    }
};