Solution2.cpp

class Solution {
public:
    unordered_set<string> s;
    string start;
    string target;

    int openLock(vector<string>& deadends, string target) {
        if (target == "0000") return 0;
        for (auto d : deadends) s.insert(d);
        if (s.count("0000")) return -1;
        this->start = "0000";
        this->target = target;
        return bfs();
    }

    int bfs() {
        unordered_map<string, int> m1;
        unordered_map<string, int> m2;
        m1[start] = 0;
        m2[target] = 0;
        queue<string> q1{{start}};
        queue<string> q2{{target}};
        while (!q1.empty() && !q2.empty()) {
            int t = q1.size() <= q2.size() ? extend(m1, m2, q1) : extend(m2, m1, q2);
            if (t != -1) return t;
        }
        return -1;
    }

    int extend(unordered_map<string, int>& m1, unordered_map<string, int>& m2, queue<string>& q) {
        for (int n = q.size(); n > 0; --n) {
            string p = q.front();
            int step = m1[p];
            q.pop();
            for (string t : next(p)) {
                if (s.count(t) || m1.count(t)) continue;
                if (m2.count(t)) return step + 1 + m2[t];
                m1[t] = step + 1;
                q.push(t);
            }
        }
        return -1;
    }

    vector<string> next(string& t) {
        vector<string> res;
        for (int i = 0; i < 4; ++i) {
            char c = t[i];
            t[i] = c == '0' ? '9' : (char) (c - 1);
            res.push_back(t);
            t[i] = c == '9' ? '0' : (char) (c + 1);
            res.push_back(t);
            t[i] = c;
        }
        return res;
    }
};