剑指offer28 字符串的排列

class Solution {
public:
    vector<string> Permutation(string str) {
        if(str.size() == 0)
            return ans;
        length = str.size();
        int begin = 0;
        Permutation(str,begin);
        //res.clear();
        set<string>::iterator it;
        for (it = res.begin(); it != res.end(); ++it)
            ans.push_back(*it);
        return ans;
    }
    void Permutation(string str,int begin){
        if(begin == length){
            res.insert(str);
            return;
        }
        for(int i = begin;i < length;i++){
            swap(str[begin],str[i]);
            Permutation(str,begin+1);
            swap(str[begin],str[i]);
        }
    }
    set<string> res;
    vector<string> ans;
    int length = 0;
};