leetcodeのWord Ladder

Word Ladder
Given two words (start and end), and a dictionary, find the length of shortest transformation sequence from start to end, such that:
Only one letter can be changed at a time
Each intermediate word must exist in the dictionary
For example, Given: start =  "hit" end =  "cog" dict =  ["hot","dot","dog","lot","log"] As one shortest transformation is  "hit" -> "hot" -> "dot" -> "dog" -> "cog" , return its length  5 . Note:
Return 0 if there is no such transformation sequence.
All words have the same length.
All words contain only lowercase alphabetic characters.
アイデア:幅優先検索を採用し、現在の単語を変換できるすべての可能な単語をキューに追加するたびに、結果が見つかるまで、またはキューが空になるまで、階層的に繰り返します.

class Solution {
public:
    int ladderLength(string start, string end, unordered_set<string> &dict) {
    	if(start.size() != end.size())return 0;
    	queue<string> path;//             
    	path.push(start);
    	dict.erase(start);
    	int count = 1;//        
    	int level = 1;//     
    	while (!path.empty())
    	{
    		string word = path.front();
    		path.pop();
    		--count;
    		int i;
    		for (i = 0;i < word.size();++i)
    		{
    			string tmp = word;
    			char ch;
    			for (ch = 'a';ch <= 'z';ch++)
    			{
    				if(tmp[i] != ch)//      
    				{
    					tmp[i] = ch;
    					if(tmp == end)return level + 1;
    					unordered_set<string>::iterator iter = dict.find(tmp);
    					if (iter != dict.end())
    					{
    						path.push(tmp);
    						
    					}	
    					dict.erase(tmp);				
    				}
    			}
    		}
    		if (count == 0)//        
    		{
    			count = path.size();
    			level++;
    		}
    	}
    	return 0;//    
    }

};