int ladderLength(string beginWord, string endWord, vector<string>& wordList) {

int size = wordList.size(), path = 0;

queue<string> q;

unordered_set<string> wordset(wordList.begin(), wordList.end()); //转化成哈希表，提高查找效率

if (wordset.find(endWord) == wordset.end()) return 0; //无法转化

unordered_map<string, bool> is_visited;

is_visited[beginWord] = 1;

q.push(beginWord);

++path;

while (!q.empty())

{

int qsize = q.size();

bool can_add = 0;

for (int i = 0; i < qsize; ++i)

{

int tmpsz = q.front().size();

for (int j = 0; j < tmpsz; ++j)

{

string tmp = q.front();

for (int alpha = 0; alpha < 26; ++alpha)

{

tmp[j] = 'a'+ alpha;

if (tmp == endWord) return path + 1;

if (is_visited[tmp] == 0 && wordset.find(tmp) != wordset.end())

{

is_visited[tmp] = 1;

q.push(tmp);

can_add = 1;

}

}

}

q.pop();

}

if (can_add) ++path;

}

return 0;

}