Word Ladder II

这道题做了好长时间都没通过，参考了http://blog.csdn.net/niaokedaoren/article/details/8884938的实现，其中在求前驱单词记录时，申请的数组要采用vector<unordered_set<string>>这样，在判断是否结束时，查找效率更高，否则用前向单词记录表查找时效率会很低。

算法中的递归找paht可以用栈来避免递归，是一个优化点。

class Solution {
public:

      vector<vector<string>> wordsladders;

    vector<vector<string>> findLadders(string start, string end, unordered_set<string> &dict) {

		  unordered_map<string,vector<string>> forword;
		  dict.insert(end);
		  InitData(forword,dict);
		  bool ret;
		  ret = GetTraces(forword,start,end,dict);
	      if(ret == false)
		  	return wordsladders;
		  vector<string> path;
		  GetPath(end,forword,path);

	      return wordsladders;

    }
	void GetPath(string& tempstr,unordered_map<string,vector<string>> &forword,vector<string>& path)
    {

	  vector<string> result;
	  path.push_back(tempstr);

	  if(forword[tempstr].empty())
	  {
	     result.resize(path.size());
         copy(path.rbegin(), path.rend(),result.begin());
		 wordsladders.push_back(result);
		 path.pop_back();
         return ;
	  }

      for(auto ite = forword[tempstr].begin(); ite!= forword[tempstr].end(); ite++)
      {
      	 GetPath(*ite,forword,path);
	  }
	  path.pop_back();

	}
	void InitData(unordered_map<string,vector<string>> &forword,unordered_set<string> &dict)
	{
	   unordered_set<string>::iterator ite;
	   pair<string,vector<string>> temp;
       for(ite = dict.begin(); ite!= dict.end(); ite++)
       {
          temp.first = *ite;
          forword.insert(temp);
	   }

	}
	bool GetTraces(unordered_map<string,vector<string>> &forword,string& start,string& end,unordered_set<string> &dict)
	{
	      vector<unordered_set<string> > buf(2);
		  int cur,pre;
          cur =0;
		  pre =1;
		  string tempstr;
		  buf[0].insert(start);
		  while(1)
		  {
			   cur = !cur;
			   pre = !pre;
	           for(auto ite = buf[pre].begin(); ite != buf[pre].end(); ite++)
	           {
	               dict.erase(*ite);
			   }
			   for(auto ite = buf[pre].begin(); ite!= buf[pre].end(); ite++)
			   {
	               for(int i = 0; i < (*ite).size(); i++)
			       {
			           tempstr = *ite;
			           for(int j=0; j< 26;j++)
			           {

			             if(‘a‘+j == tempstr[i])
						  continue ;
						 tempstr[i] = ‘a‘+j;
						 if(dict.find(tempstr) == dict.end())
						   continue;
						 forword[tempstr].push_back(*ite);
						 buf[cur].insert(tempstr);
					   }

				  }

			 }
			 buf[pre].clear();
			 if(buf[cur].size() == 0)
			 	return false;
			 if(buf[cur].count(end))
			 	 return true;

		}

   }

};