面试题 08.08：有重复字符串的排列组合

输入：S = "qqe"
 输出：["eqq","qeq","qqe"]

输入：S = "ab"
 输出：["ab", "ba"]

class Solution {
public:
    string path;
    vector<string> res;
    void backtracing(string& S,vector<bool>& used){
        if(path.size()==S.size()){
            res.push_back(path);
            return;
        }
        for(int i=0;i<S.size();i++){
            if(i>0&&S[i]==S[i-1]&used[i-1]==false) continue;
            if(used[i]==false){
                used[i]=true;
                path.push_back(S[i]);
                backtracing(S,used);
                path.pop_back();
                used[i]=false;
            }
        }
    }   
    vector<string> permutation(string S) {
        sort(S.begin(),S.end());
        vector<bool> used(S.size(),false);
        backtracing(S,used);
        return res;
    }
};