链接：剑指 Offer II 109. 开密码锁

题解：

 

class Solution {
public:
    int openLock(vector<string>& deadends, string target) {
        if (target == "0000") {
            return 0;
        }

        unordered_set<string> dead(deadends.begin(), deadends.end());
        if (dead.count("0000")) {
            return -1;
        }

        auto num_prev = [](char x) -> char {
            return (x == '0' ? '9' : x - 1);
        };
        auto num_succ = [](char x) -> char {
            return (x == '9' ? '0' : x + 1);
        };

        // 枚举 status 通过一次旋转得到的数字
        auto get = [&](string& status) -> vector<string> {
            vector<string> ret;
            for (int i = 0; i < 4; ++i) {
                char num = status[i];
                status[i] = num_prev(num);
                ret.push_back(status);
                status[i] = num_succ(num);
                ret.push_back(status);
                status[i] = num;
            }
            return ret;
        };

        queue<pair<string, int>> q;
        q.emplace("0000", 0);
        unordered_set<string> seen = {"0000"};

        while (!q.empty()) {
            auto [status, step] = q.front();
            q.pop();
            for (auto&& next_status: get(status)) {
                if (!seen.count(next_status) && !dead.count(next_status)) {
                    if (next_status == target) {
                        return step + 1;
                    }
                    q.emplace(next_status, step + 1);
                    seen.insert(move(next_status));
                }
            }
        }

        return -1;
    }
};