class Solution {public:    int ladderLength(string start, string end, unordered_set
    
      &dict) {        // IMPORTANT: Please reset any member data you declared, as        // the same Solution instance will be reused for each test case.        //BFS遍历找到的第一个匹配就是最短转换,空字符串是层与层之间的分隔标志        queue
     
       Q;        Q.push(start); Q.push("");        int res = 1;        while(Q.empty() == false)        {            string str = Q.front();            Q.pop();            if(str != "")            {                int strLen = str.length();                for(int i = 0; i < strLen; i++)                {                    char tmp = str[i];                    for(char c = 'a'; c <= 'z'; c++)                    {                        if(c == tmp)continue;                        str[i] = c;                        if(str == end)return res+1;                        if(dict.find(str) != dict.end())                        {                            Q.push(str);                            dict.erase(str);                        }                    }                    str[i] = tmp;                }            }            else if(Q.empty() == false)            {//到达当前层的结尾，并且不是最后一层的结尾                res++;                Q.push("");            }        }        return 0;    }};