355. Design Twitter

Design a simplified version of Twitter where users can post tweets, follow/unfollow another user and is able to see the 10 most recent tweets in the user's news feed. Your design should support the following methods:

1. postTweet(userId, tweetId): Compose a new tweet.
2. getNewsFeed(userId): Retrieve the 10 most recent tweet ids in the user's news feed. Each item in the news feed must be posted by users who the user followed or by the user herself. Tweets must be ordered from most recent to least recent.
3. follow(followerId, followeeId): Follower follows a followee.
4. unfollow(followerId, followeeId): Follower unfollows a followee.

Example:

Twitter twitter = new Twitter();

// User 1 posts a new tweet (id = 5).
twitter.postTweet(1, 5);

// User 1's news feed should return a list with 1 tweet id -> [5].
twitter.getNewsFeed(1);

// User 1 follows user 2.
twitter.follow(1, 2);

// User 2 posts a new tweet (id = 6).
twitter.postTweet(2, 6);

// User 1's news feed should return a list with 2 tweet ids -> [6, 5].
// Tweet id 6 should precede tweet id 5 because it is posted after tweet id 5.
twitter.getNewsFeed(1);

// User 1 unfollows user 2.
twitter.unfollow(1, 2);

// User 1's news feed should return a list with 1 tweet id -> [5],
// since user 1 is no longer following user 2.
twitter.getNewsFeed(1);

首先。。。我没见过Twitter长什么样好吗。。一点都不友好

这里核心的部分实际上是一个多路的数据归并问题。

以两个的情况作为例子：A发了一大堆微博，存在链表里面，B也发了一大堆，如果调用到了follow，那么A和B就需要取前10个，

需要归并一下，把两者的最近放在前面。

多路归并，就设置相应的指针，对每一路取当前指针位置的值比较，不断找出最小的直到没有数据或者满10个为止，

每轮找到最小值以后对应的该路指针推进一位。

public class Twitter
{

	/** Initialize your data structure here. */
	HashMap<Integer, LinkedList<Integer>> hashmap;
	HashMap<Integer, HashSet<Integer>> followhashmap;
	HashMap<Integer, Integer> seqhashmap;
	HashSet<Integer> rethashset;
	public Twitter()
	{
		hashmap=new HashMap<>(1024);
		followhashmap=new HashMap<>();
		seqhashmap=new HashMap<>(1024);
	}

	/** Compose a new tweet. */
	public void postTweet(int userId, int tweetId)
	{
		if(!hashmap.containsKey(userId))
			hashmap.put(userId, new LinkedList<Integer>());
	
		hashmap.get(userId).addFirst(tweetId);
		
		seqhashmap.put(tweetId, seqhashmap.size());
	}

	/**
	 * Retrieve the 10 most recent tweet ids in the user's news feed. Each item
	 * in the news feed must be posted by users who the user followed or by the
	 * user herself. Tweets must be ordered from most recent to least recent.
	 */
	public List<Integer> getNewsFeed(int userId)
	{
		if(!followhashmap.containsKey(userId))
		{
			LinkedList<Integer> llist=hashmap.get(userId);
			if(llist==null)
				return new ArrayList<>();
			else if(llist.size()<=10)
				return new ArrayList<>(llist);
			else {
				ArrayList<Integer> rarraylist=new ArrayList<>();
				int cnt=0;
				for(int num : llist)
					{
						rarraylist.add(num);
						cnt++;
						if(cnt==10)
							break;
					}
				return rarraylist;
			}
		}
		
		
		LinkedList<Integer>[] userarraylist=new LinkedList[followhashmap.get(userId).size()+1];
		ArrayList<Integer> retlist=new ArrayList<>();
		userarraylist[0]=hashmap.get(userId);
		int cnt=1;
		for(int n : followhashmap.get(userId))
			userarraylist[cnt++]=hashmap.get(n);
		
		int[] point=new int[cnt];
		boolean canstop=false;
		while(!canstop)
		{
			int maxindex=-1;
			int max=Integer.MIN_VALUE;
			for(int i=0;i<cnt;i++)
			{
				LinkedList<Integer> llist=userarraylist[i];
				if(llist!=null&&point[i]<llist.size())
				{	
					int num=seqhashmap.get(llist.get(point[i]));
					if(num>max)
					{
						maxindex=i;
						max=num;
					}
				}
			}
			
			if(maxindex==-1)
				break;
			retlist.add(userarraylist[maxindex].get(point[maxindex]));
			point[maxindex]++;
			
			if(retlist.size()>=10)
				break;
			
			canstop=true;
			for(int i=0;i<cnt;i++)
			{
				if(userarraylist[i]==null)
					continue;
				if(point[i]<userarraylist[i].size())
				{
					canstop=false;
					break;
				}
			}
		}
		
		return retlist;
		
	}

	/**
	 * Follower follows a followee. If the operation is invalid, it should be a
	 * no-op.
	 */
	public void follow(int followerId, int followeeId)
	{
		if(followerId==followeeId)
			return ;
		
		if(!followhashmap.containsKey(followerId))
			followhashmap.put(followerId, new HashSet<Integer>());
		
		followhashmap.get(followerId).add(followeeId);
		
	}

	/**
	 * Follower unfollows a followee. If the operation is invalid, it should be
	 * a no-op.
	 */
	public void unfollow(int followerId, int followeeId)
	{
		if(!followhashmap.containsKey(followerId))
			return ;
		if(!followhashmap.get(followerId).contains(followeeId))
			return ;
		
		followhashmap.get(followerId).remove(followeeId);
	}
}

/**
 * Your Twitter object will be instantiated and called as such:
 * Twitter obj = new Twitter();
 * obj.postTweet(userId,tweetId);
 * List<Integer> param_2 = obj.getNewsFeed(userId);
 * obj.follow(followerId,followeeId);
 * obj.unfollow(followerId,followeeId);
 */