BFS word ladder problem

wordLadder.cc

class Solution {
public:
    int ladderLength(string beginWord, string endWord, vector<string>& wordList) {
        queue<string> q;
        unordered_set<string> m, dict;
        q.push(beginWord);
        m.insert(beginWord);
        for(const auto & it : wordList)
            dict.insert(it);   
        int level = 1;        
        while(!q.empty()){
            queue<string> nex;
            while(!q.empty()){
                string s = q.front();
                q.pop();
                if(endWord == s) return level;
                for(int i = 0; i < s.size() ; i++){
                    string t = s;
                    for(int j = 0; j < 26; j++){
                        t[i] = 'a'+j;                                                
                        if(dict.find(t) != dict.end() && m.find(t) == m.end()) {
                            nex.push(t);                                
                            m.insert(t);
                        }
                    }
                }
            }
            q = nex;
            level++;
        }
        return 0;
    }
};