Solution.cpp

class Solution {
public:
    int openLock(vector<string>& deadends, string target) {
        unordered_set<string> s(deadends.begin(), deadends.end());
        if (s.count("0000")) return -1;
        if (target == "0000") return 0;
        queue<string> q{{"0000"}};
        s.insert("0000");
        int ans = 0;
        while (!q.empty()) {
            ++ans;
            for (int n = q.size(); n > 0; --n) {
                string p = q.front();
                q.pop();
                for (string t : next(p)) {
                    if (target == t) return ans;
                    if (!s.count(t)) {
                        q.push(t);
                        s.insert(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;
    }
};