链接：. - 力扣（LeetCode）

题解：堆和hash表

class Solution {
public:
    int getKthMagicNumber(int k) {
        if (k <= 0) {
            return -1;
        }
        if (k == 1) {
            return 1;
        }
        std::unordered_set<int64_t> table;
        std::vector<int64_t> nums{3, 5, 7};
        std::priority_queue<int64_t, std::vector<int64_t>, std::greater<int64_t>> que;
        que.push(1);
        table.insert(1);
        int result = 1;
        for (int i = 0; i < k; ++i) {
            auto top = que.top();
            result = top;
            que.pop();
            for (auto& num : nums) {
                int64_t next_num = num * top;
                if (table.find(next_num) != table.end()) {
                    continue;
                }
                que.push(next_num);
                table.insert(next_num);
            }
        }

        return result;
    }
};