HDU-3639-Hawk-and-Chicken（强连通，缩点，DFS）

链接：https://vjudge.net/problem/HDU-3639

题意：

有n个小朋友在一个班级中，现在要选择班长。收集了小朋友们的意见，一条意见表示为A认为B合适。这个是具备传递性的，A认为B合适，B认为C合适。那么A也会认为C合适。 
现在需要提供一份候选人名单，这里面的人，是被最多的人，认为合适的。

思路：

tarjan，缩点，再根据缩点之后的点建立反向的新图，用来求每个人的的票数。

不过在找得票数的时候用记忆化搜索wa了，不知道为啥

代码：

#include <iostream>
#include <memory.h>
#include <string>
#include <istream>
#include <sstream>
#include <vector>
#include <stack>
#include <algorithm>
#include <map>
#include <queue>
#include <math.h>
#include <cstdio>
#include <set>
#include <iterator>
#include <cstring>
using namespace std;

typedef long long LL;
const int MAXN = 5e3+10;
const int INF = 0x3f3f3f3f;

vector<int> G[MAXN];
vector<int> Gr[MAXN];
stack<int> St;
int Dfn[MAXN], Low[MAXN];
int Vis[MAXN], Dis[MAXN];
int Fa[MAXN], Fans[MAXN];
int Num[MAXN];
int n, m;
int times, cnt;

void Init()
{
    for (int i = 1;i <= n;i++)
        G[i].clear(), Fa[i] = i, Gr[i].clear();
    memset(Dfn, 0, sizeof(Dfn));
    memset(Low, 0, sizeof(Low));
    memset(Vis, 0, sizeof(Vis));
    memset(Dis, 0, sizeof(Dis));
    memset(Num, 0, sizeof(Num));
    memset(Fans, -1, sizeof(Fans));
    times = cnt = 0;
}

void Tarjan(int x)
{
    Dfn[x] = Low[x] = ++times;
    Vis[x] = 1;
    St.push(x);
    for (int i = 0;i < G[x].size();i++)
    {
        int node = G[x][i];
        if (Dfn[node] == 0)
        {
            Tarjan(node);
            Low[x] = min(Low[x], Low[node]);
        }
        else if (Vis[node] == 1)
            Low[x] = min(Low[x], Dfn[node]);
    }
    if (Low[x] == Dfn[x])
    {
        cnt++;
        Num[cnt] = 0;
        while (St.top() != x)
        {
            Num[cnt]++;
            Fa[St.top()] = cnt;
            Vis[St.top()] = 0;
            St.pop();
        }
        Num[cnt]++;
        Fa[St.top()] = cnt;
        Vis[St.top()] = 0;
        St.pop();
    }
}

int GetFans(int x)
{
    Vis[x] = 1;
    int sum = 0;
    for (int i = 0;i < Gr[x].size();i++)
    {
        int node = Gr[x][i];
        if (Vis[node] == 1)
            continue;
        sum += Num[node] + GetFans(node);
    }
    return sum;
}

int main()
{
    int t, cn = 0;
    cin >> t;
    while (t--)
    {
        cin >> n >> m;
        Init();
//            cin >> Cost[i];
        int l, r;
        for (int i = 1;i <= m;i++)
        {
            scanf("%d%d", &l, &r);
            l++, r++;
//            cin >> l >> r;
            G[l].push_back(r);
        }
        for (int i = 1;i <= n;++i)
            if (!Dfn[i])
                Tarjan(i);
        for (int i = 1;i <= n;i++)
        {
            for (int j = 0;j < G[i].size();j++)
            {
                int node = G[i][j];
                int l = Fa[i], r = Fa[node];
                if (l != r)
                {
                    ++Dis[l];
                    Gr[r].push_back(l);
                }
            }
        }
        int number = -1, is = 0;
        for (int i = 1;i <= cnt;i++)
        {
            if (Dis[i] == 0)
            {
                memset(Vis, 0, sizeof(Vis));
                Fans[i] = Num[i]-1 + GetFans(i);
                number = max(number, Fans[i]);
            }
        }
        cout << "Case " << ++cn << ": " << number << endl;
        for (int i = 1;i <= n;i++)
        {
            if (Fans[Fa[i]] == number)
            {
                if (is++ == 0)
                    cout << i-1;
                else
                    cout << ' ' << i-1 ;
            }
        }
        cout << endl;
    }

    return 0;
}