Word Ladder II

class Solution {public:    vector<vector<string>> findLadders(string start, string end, unordered_set<string> &dict) {          vector<vector<string> >ans;          if(start == end) return ans;          unordered_set<string>current , next;          unordered_set<string> flag;          unordered_map<string,vector<string> > father;                    current.insert(start);          bool found = false;                   while(!current.empty() && !found) {              //expand              for(const auto &x : current) {                  flag.insert(x);              }                            for(auto x : current) {                  for(int i = 0 ; i < x.size() ; ++i) {                      for(int j = 'a' ; j <= 'z' ; ++j) {                          if(x[i] == j) continue;                          string tmp = x;                          tmp[i] = j;                          if(tmp == end) found = true;                          if(dict.find(tmp) != dict.end() && flag.find(tmp) == flag.end()) {                              next.insert(tmp);                              father[tmp].push_back(x);                          }                     }                  }              }              //end expand                            current.clear();              swap(current, next);          }          //start foudn father                    if(found) {              vector<string> c;              dfs(ans , father , c , start , end);          }          return ans;    }private:    void dfs(vector<vector<string> >&ans,              unordered_map<string,vector<string> >& father ,             vector<string>& c ,              string& start ,             string& now) {                         c.push_back(now);        if(now == start) {            ans.push_back(c);            reverse(ans.back().begin() , ans.back().end());            c.pop_back();            return;        }        auto que = father.find(now) -> second;        for(auto& x : que) {            dfs(ans , father , c , start , x);        }        c.pop_back();    }};