a1076 Forwards on Weibo （DFS、BFS）

1076 Forwards on Weibo (30 分)

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 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 trigger, 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

 刚开始使用DFS，错了两个测试点，后面想想，DFS在这道题好像没有BFS方便

DFS：

#include 
#include "vector"
using namespace std;
const int maxx = 1100;
int n,l;
bool vis[maxx];
vector fans[maxx],fowwards;
void DFS(int start,int depth){
    if (depth > l) return;
    vis[start] = true;
    if (depth <= l){
        fowwards.push_back(start);
    }
    for (int i = 0; i < fans[start].size(); ++i) {
        int u = fans[start][i];
        if (!vis[u]){
            DFS(u,depth+1);
        }
    }
}
int main() {
    cin>>n>>l;
    int temp,aa;
    for (int i = 1; i <= n; ++i) {
        cin>>temp;
        for (int j = 0; j < temp; ++j) {
            cin>>aa;
            fans[aa].push_back(i);
        }
    }
    cin>>temp;
    for (int i = 0; i < temp; ++i) {
        fowwards.clear();
        fill(vis,vis+maxx,false);
        cin>>aa;
        DFS(aa,0);
        cout<

DFS在遇到下面这种情况时，会出错。 

 BFS把每层能到达的节点先放进队列中，不会出现BFS那样的问题；

BFS:

#include "iostream"
#include "vector"
#include "queue"
using namespace std;
int n,l,forwrds;
bool vis[1100];
vector fans[1100];
struct node{
    int id,level;
};
int BFS(int u){
    queue m;
    node a = {u,0};
    m.push(a);
    vis[u] = true;
    while (!m.empty()){
        node top = m.front();
        m.pop();
        int a = top.id;
        for (int i = 0; i < fans[a].size(); ++i) {
            node b = {fans[a][i],top.level+1};
            if (!vis[b.id] && b.level <= l){
                m.push(b);
                vis[b.id] = true;
                forwrds++;
            }
        }
    }
    return forwrds;
}
int main(){
    cin>>n>>l;
    int temp,aa;
    for (int i = 1; i <= n; ++i) {
        cin>>temp;
        for (int j = 0; j < temp; ++j) {
            cin>>aa;
            fans[aa].push_back(i);
        }
    }
    cin>>temp;
    for (int i = 0; i < temp; ++i) {
        forwrds=0;
        fill(vis,vis+1100, false);
        cin>>aa;
        int ans = BFS(aa);
        cout<