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
", ans);
	return 0;
}