1311.cpp

class Solution {
  public:
    vector<string> watchedVideosByFriends(vector<vector<string>> &watchedVideos, vector<vector<int>> &adj,
                                          int id, int level) {
        int n = adj.size();
        vector<bool> visited(n, false);
        queue<int> q;

        q.push(id);
        visited[id] = true;
        for (int lvl = 0; lvl != level; lvl++) {
            for (int k = q.size(); k > 0; k--) {
                int id = q.front();
                q.pop();
                for (int c : adj[id]) {
                    if (!visited[c]) {
                        visited[c] = true;
                        q.push(c);
                    }
                }
            }
        }

        unordered_map<string, int> freq;
        vector<pair<int, string>> vec;
        vector<string> res;

        for (; !q.empty(); q.pop())
            for (auto &st : watchedVideos[q.front()])
                freq[st]++;

        for (auto &[k, v] : freq)
            vec.push_back({v, k});
        sort(vec.begin(), vec.end());
        for (auto &[_, title] : vec)
            res.push_back(title);
        return res;
    }
};