剑指offer——字符串的排列

#include <string>
#include <vector>
#include <set>
#include <algorithm>
using namespace std;

class Solution {
    public:
        vector<string> Permutation(string str) {
            char* ch = (char*)str.c_str();
            int len = str.length();
            vector<string> ans; 
            set<string> s;
            if(str == ""){
                return ans;
            }
            dfs(s,str,len,0);
            for(set<string>::iterator it = s.begin() ; it != s.end() ; it++){
                ans.push_back(*it);
            }
            sort(ans.begin(),ans.end());
            return ans;
        }

        void dfs(set<string>& s,string str,int len,int cnt){
            if(cnt == len){
                s.insert(str);
            }else{
                for(int i = cnt ; i < len ; i++){
                    char tmp = str[cnt];
                    str[cnt] =str[i];
                    str[i] = tmp;
                    dfs(s,str,len,cnt+1);
                    tmp = str[cnt];
                    str[cnt] =str[i];
                    str[i] = tmp;
                }
            }
        }
};