1076. Forwards on Weibo (30)

题目：

Weibo is known as the Chinese version of Twitter. One user on Weibo may have many followers, and may follow many other users as well. Hence a social network is formed with followers relations. When a user makes a post on Weibo, all his/her followers can view and forward his/her post, which can then be forwarded again by their followers. Now given a social network, you are supposed to calculate the maximum potential amount of forwards for any specific user, assuming that only L levels of indirect followers are counted.

Input Specification:

Each input file contains one test case. For each case, the first line contains 2 positive integers: N (<=1000), the number of users; and L (<=6), the number of levels of indirect followers that are counted. Hence it is assumed that all the users are numbered from 1 to N. Then N lines follow, each in the format:

M[i] user_list[i]

where M[i] (<=100) is the total number of people that user[i] follows; and user_list[i] is a list of the M[i] users that are followed by user[i]. It is guaranteed that no one can follow oneself. All the numbers are separated by a space.

Then finally a positive K is given, followed by K UserID's for query.

Output Specification:

For each UserID, you are supposed to print in one line the maximum potential amount of forwards this user can triger, assuming that everyone who can view the initial post will forward it once, and that only L levels of indirect followers are counted.

Sample Input:

7 3
3 2 3 4
0
2 5 6
2 3 1
2 3 4
1 4
1 5
2 2 6

Sample Output:

4
5

注意：

1、input中的list是当前user关注的人，也就是说我们在建立关注表的时候应该反过来，当前user是list中的user的粉丝。

2、使用bfs搜索，注意搜索的层数是有限制的。

代码：

//1076
#include<iostream>
#include<vector>

using namespace std;
vector<vector<int>>usr;//user's follower lists
//n:the number of users;
//L:the number of levels of indirect followers that are counted
int n,L;

int bfs(int id)
{//bfs search
	int count=0;//maximum amount of forwards
	vector<int>curlevel,nextlevel;
	vector<int>visited;//visited flag
	visited.assign(n+1,0);
	visited[id]=1;
	curlevel.push_back(id);
	for(int i=0;i<L;++i)
	{//search only L level
		nextlevel.clear();
		for(int j=0;j<curlevel.size();++j)
		{
			for(int k=0;k<usr[curlevel[j]].size();++k)
				if(!visited[usr[curlevel[j]][k]])
				{
					nextlevel.push_back(usr[curlevel[j]][k]);
					visited[usr[curlevel[j]][k]]=1;
					++count;
				}
		}
		curlevel=nextlevel;
	}
	return count;
}

int main()
{
	scanf("%d%d",&n,&L);
	usr.resize(n+1);
	
	for(int i=1;i<=n;++i)
	{
		int m,temp;
		scanf("%d",&m);
		for(int j=0;j<m;++j)
		{//i is the follower of temp
			scanf("%d",&temp);
			usr[temp].push_back(i);
		}
	}
	int k;
	scanf("%d",&k);
	for(int i=0;i<k;++i)
	{
		int id;
		scanf("%d",&id);
		printf("%d\n",bfs(id));
	}
	return 0;
}