127. Word Ladder

class Solution {
public:
    int ladderLength(string beginWord, string endWord, vector<string>& wordList) {
        unordered_set<string> s(wordList.begin(), wordList.end());
        if (s.find(endWord) == s.end() || beginWord == endWord) return 0;
        queue<string> q;
        q.push(beginWord);
        s.erase(beginWord);
        int lv = 1;
        while (!q.empty()) {
            lv++;
            int qs = q.size();
            while (qs-- > 0) {
                beginWord = q.front();
                q.pop();
                for (int i = 0; i < beginWord.length(); i++) {
                    char back = beginWord[i];
                    for (char c = 'a'; c <= 'z'; c++) {
                        beginWord[i] = c;
                        if (beginWord == endWord)
                            return lv;
                        if (s.find(beginWord) == s.end())
                            continue;
                        s.erase(beginWord);
                        q.push(beginWord);
                    }
                    beginWord[i] = back;
                }
            }
        }
        return 0;
    }
};