word ladder I

class Solution {
public:
    /*
     * @param start: a string
     * @param end: a string
     * @param dict: a set of string
     * @return: An integer
     */
    int ladderLength(string &start, string &end, unordered_set<string> &dict) {
        dict.insert(start);
        dict.insert(end);
        queue<string> q;
        q.push(start);
        int step = 0;
        while (!q.empty()) {
            int size = q.size();
            ++step;
            for (int i = 0; i < size; ++i) {
                string str = q.front(); q.pop();
                if (str == end) return step;
                for (string neighbor : getNeighbors(str, dict)) {
                    q.push(neighbor);
                    dict.erase(neighbor);
                }
            }
        }
        return -1;
    }
    vector<string> getNeighbors(string& curr, unordered_set<string>& dict) {
        vector<string> ans;
        int len = curr.size();
        for (int i = 0; i < len; ++i) {
            for (char c = 'a'; c <= 'z'; ++c) {
                if (curr[i] == c) continue;
                string str = curr.substr(0, i) + c + curr.substr(i+1, curr.size() - i - 1);
                if (dict.find(str) == dict.end()) continue;
                ans.push_back(str);
            }
        }
        return ans;
    }
};