本题源自LeetCode

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大神的想法

http://www.cnblogs.com/ShaneZhang/p/3748494.html

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   vector<vector<string>> result;
    vector<string> temp;
public:
    vector<vector<string>> findLadders(string start, string end, unordered_set<string> &dict) {
        temp.clear();
        result.clear();
        
        unordered_set<string> current;
        unordered_set<string> next;
        unordered_map<string,unordered_set<string>> path;
        unordered_set<string> ret=dict;
        //从字典中删除单词
        if(ret.count(start)>0)
            ret.erase(start);
        current.insert(start);
        
        while(current.count(end)==0&&ret.size()>0){
            for(auto p=current.begin();p!=current.end();p++){
                string word=*p;
                for(int i=0;i<start.length();i++){
                    for(char j='a';j<='z';j++){
                        string tmp=word;
                        if(tmp[i]==j)
                            continue;
                        tmp[i]=j;
                        if(ret.count(tmp)>0){
                            next.insert(tmp);
                            //构建反向邻接链表
                            path[tmp].insert(word);
                        }
                    }
                }
            }
            if(next.empty())
                break;
            //从字典中删除这个层的单词避免重复
            for(auto p=next.begin();p!=next.end();p++){
                ret.erase(*p);
            }
            current=next;
            next.clear();
        }
        if(current.count(end)>0)
            GeneratePath(end,start,path);
        return result;
    }
    void GeneratePath(string& start,string& end,unordered_map<string,unordered_set<string>>& path){
        temp.push_back(start);
        if(start==end){
            reverse(temp.begin(),temp.end());
            result.push_back(temp);
            reverse(temp.begin(),temp.end());
            return;
        }
       for(auto it = path[start].begin(); it != path[start].end(); ++it){
           string str=*it;
           GeneratePath(str, end, path);
           temp.pop_back();
       }
    }