LeetCode 1500. Design a File Sharing System（哈希map+优先队列）

Input:
["FileSharing","join","join","join","request","request","leave","request","leave","join"]
[[4],[[1,2]],[[2,3]],[[4]],[1,3],[2,2],[1],[2,1],[2],[[]]]
Output:
[null,1,2,3,[2],[1,2],null,[],null,1]
Explanation:
FileSharing fileSharing = new FileSharing(4); 
// We use the system to share a file of 4 chunks.

fileSharing.join([1, 2]);    
// A user who has chunks [1,2] joined the system, assign id = 1 to them and return 1.

fileSharing.join([2, 3]);    
// A user who has chunks [2,3] joined the system, assign id = 2 to them and return 2.

fileSharing.join([4]);       
// A user who has chunk [4] joined the system, assign id = 3 to them and return 3.

fileSharing.request(1, 3);   
// The user with id = 1 requested the third file chunk, as only the user with id = 2 has the file, return [2] . Notice that user 1 now has chunks [1,2,3].

fileSharing.request(2, 2);   
// The user with id = 2 requested the second file chunk, users with ids [1,2] have this chunk, thus we return [1,2].

fileSharing.leave(1);        
// The user with id = 1 left the system, all the file chunks with them are no longer available for other users.

fileSharing.request(2, 1);   
// The user with id = 2 requested the first file chunk, no one in the system has this chunk, we return empty list [].

fileSharing.leave(2);        
// The user with id = 2 left the system.

fileSharing.join([]);        
// A user who doesn't have any chunks joined the system, assign id = 1 to them and return 1. Notice that ids 1 and 2 are free and we can reuse them.
 

Constraints:
1 <= m <= 10^5
0 <= ownedChunks.length <= min(100, m)
1 <= ownedChunks[i] <= m
Values of ownedChunks are unique.
1 <= chunkID <= m
userID is guaranteed to be a user in the system if you assign the IDs correctly. 
At most 10^4 calls will be made to join, leave and request.
Each call to leave will have a matching call for join.

class FileSharing {
	priority_queue<int,vector<int>,greater<int>> numCanBeUsed;//小号优先
	unordered_map<int,set<int>> user_file;
	unordered_map<int,set<int>> file_user;
public:
    FileSharing(int m) {
    	for(int i = 1; i <= m; ++i)
    		numCanBeUsed.push(i);
    }
    
    int join(vector<int> ownedChunks) {
    	if(numCanBeUsed.empty()) return -1;
    	int id = numCanBeUsed.top();//取号
    	numCanBeUsed.pop();
    	for(int f : ownedChunks)
    	{
    		user_file[id].insert(f);
    		file_user[f].insert(id);
    	}
    	return id;
    }
    
    void leave(int userID) {
    	numCanBeUsed.push(userID);//号放回
    	for(int f : user_file[userID])
            file_user[f].erase(userID);
    	user_file.erase(userID);
    }
    
    vector<int> request(int userID, int chunkID) {
    	vector<int> ans(file_user[chunkID].begin(), file_user[chunkID].end());
        if(!ans.empty() && !user_file[userID].count(chunkID))
        {	//如果文件存在，且 该用户没有该文件
            user_file[userID].insert(chunkID);
            file_user[chunkID].insert(userID);
        }
        return ans;
    }
};