LuoGu-P2863牛的舞会The Cow Prom[tarjan 缩点模板]

传送门：https://www.luogu.org/problemnew/show/P2863

思路：tarjan模板题，之前会的tarjan，一直想学缩点到底是什么操作，发现就是把同组的放在一个数组里（并查集可能又会有），或者重新建图；不知道为什么百度不到讲缩点的blog。

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <list>
#include <iterator>

#define lson (l , mid , rt << 1)
#define rson (mid + 1 , r , rt << 1 | 1)
#define debug(x) cerr << #x << " = " << x << "\n";
#define pb push_back
typedef long long ll;

using namespace std;


const int maxn = 10009;
int n,m;
vector<int >mp[maxn];
int dfn[maxn],low[maxn],ind;
int stk[maxn],p;
bool vis[maxn];
int ans = 0, cnt = 0;
int q[maxn];
void tarjan(int x)
{   
    ind++;
    dfn[x] = low[x] = ind;
    vis[x] = true;
    stk[++p] = x;

    for(int i=0;i<mp[x].size();i++)
    {
        int to = mp[x][i];
        if(dfn[to]==0)
        {
            tarjan(to);
            low[x] = min(low[x],low[to]);
        }
        else if(vis[to])
        {
            low[x] = min(low[x],dfn[to]);
        }
    }
    if(low[x]==dfn[x])
    {
          cnt++;
         
        while(1)
        {
         
            int tmp = stk[p--];
            q[cnt]++;
            vis[tmp]=false;
            if(tmp==x)break;
        }
    }
}
int main(){
    scanf("%d%d", &n,&m);
    for(int i=1; i<=m; i++)
    {
        int u,v;
        scanf("%d%d", &u,&v);
        mp[u].pb(v);
    }
    for(int i=1; i<=n; i++)
    {
        if(dfn[i]==0)
        {
            tarjan(i);
        }
    }
    for(int i=1;i<=cnt;i++)
        if(q[i]>1)ans++;
    printf("%d\n",ans);


    return 0;
}