word ladder

I

class Solution {
public:
    int ladderLength(string start, string end, unordered_set<string> &dict) {
        unordered_set<string> added;
        queue<string> q;
        int ret = 0;
        int lev1 = 1, lev2 = 0;
        q.push(start);
        added.insert(start);
        while(!q.empty()) 
        {
            string s = q.front(); q.pop();
            --lev1;
            for(int i = 0; i < s.length(); ++i) 
            {
                for(int j = 0; j < 26; ++j) 
                {
                    string t = s;
                    t[i] = 'a' + j;
                    if(t != s)
                    {
                        if(t == end) return ret+2;
                        if(dict.find(t) != dict.end() && added.find(t) == added.end()) {
                            q.push(t);
                            added.insert(t);
                            ++lev2;
                        }
                    }
                }
            }
            if(lev1 == 0) {
                ++ret;
                lev1 = lev2;
                lev2 = 0;
            }
        }
     return 0;
    }
};