洛谷 P2863 [USACO06JAN]牛的舞会The Cow Prom

牛的舞会（tarjan板子题）

tarjan算法学习过程中必做的一道板子题。

#include <iostream>
#include <cstring>
#include <cstdio>
#include <stack>
#include <queue>
using namespace std;
//Mystery_Sky
//
#define maxn 1000010
#define INF 0x3f3f3f3f
struct Edge{
    int to;
    int next;
}edge[maxn];
stack <int> st;
int low[maxn], dfn[maxn], vis[maxn];
int head[maxn], cnt;
int scc, ans, tot, num[maxn];
int n, m, u, v;
inline void add_edge(int u, int v)
{
    edge[++cnt].to = v;
    edge[cnt].next = head[u];
    head[u] = cnt;
}

void tarjan(int u)
{
    dfn[u] = low[u] = ++tot;
    st.push(u);
    vis[u] = 1;
    for(int i = head[u]; i; i = edge[i].next) {
        int v = edge[i].to;
        if(!dfn[v]) {
            tarjan(v);
            low[u] = min(low[u], low[v]);
        }
        else if(vis[v]) low[u] = min(low[u], dfn[v]);
    }
    if(dfn[u] == low[u]) {
        int j;
        scc++;
        do {
            j = st.top();
            st.pop();
            vis[j] = 0;
            num[scc]++; 
        } while(j != u);
    } 
}

int main() {
    scanf("%d%d", &n, &m);
    for(int i = 1; i <= m; i++) {
        scanf("%d%d", &u, &v);
        add_edge(u, v);
    }
    for(int i = 1; i <= n; i++) 
        if(!dfn[i]) tarjan(i);
    for(int i = 1; i <= scc; i++) {
        if(num[i] > 1) ans++;
    }
    printf("%d\n", ans);
    return 0;
}