单词接龙 II

给出两个单词（start和end）和一个字典，找出所有从start到end的最短转换序列

比如：

1. 每次只能改变一个字母。
   
2. 变换过程中的中间单词必须在字典中出现。

样例

给出数据如下：

start = "hit"

end = "cog"

dict = ["hot","dot","dog","lot","log"]

返回

[

    ["hit","hot","dot","dog","cog"],

    ["hit","hot","lot","log","cog"]

  ]

注意

• 所有单词具有相同的长度。
  
• 所有单词都只包含小写字母。

class Solution {public:    /**      * @param start, a string      * @param end, a string      * @param dict, a set of string      * @return a list of lists of string      */    vector<vector<string>> findLadders(string start, string end, unordered_set<string> &dict) {        // write your code here        vector<vector<string> > result;        set<string> cur;        cur.insert(start);        set<string> next;        map<string, set<string> > parents;        unordered_set<string> unused = dict;        unused.insert(end);        visit(cur, end, next, unused, parents);        if (parents.find(end) != parents.end())        {            vector<string> buf;            generatePath(start, end, parents, buf, result);        }        return result;    }private:    void visit(set<string> &cur, string &end, set<string> &next,                unordered_set<string> &unused, map<string, set<string> > &parents)    {        while (parents.find(end) == parents.end() && !unused.empty())        {            next.clear();            for (set<string>::iterator it = cur.begin(); it != cur.end(); it++)            {                string word = *it;                string temp = *it;                for (int i = 0; i < (*it).length(); i++)                {                    for (char a = 'a'; a <= 'z'; a++)                    {                        temp = *it;                        if (a != word[i])                        {                            temp[i] = a;                            if (unused.find(temp) != unused.end())                            {                                parents[temp].insert(word);                                next.insert(temp);                            }                        }                    }                }            }            if (next.empty())            {                return;            }            cur = next;            for (set<string>::iterator it = next.begin(); it != next.end(); it++)            {                unused.erase(*it);            }        }    }    void generatePath(string &start, string cur, map<string, set<string> > &parents,                   vector<string> &buf, vector<vector<string> > &result)    {        buf.insert(buf.begin(), cur);        if (cur == start)        {            result.push_back(buf);        }        else        {            for (set<string>::iterator it = parents[cur].begin(); it != parents[cur].end(); it++)            {                generatePath(start, *it, parents, buf, result);            }        }        buf.erase(buf.begin());    }};