#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;

const int N = 10005;

vector<int>e[N];

int dfn[N], low[N], inn[N], outt[N], scc[N], cnt, tot, instk[N], stk[N], top, siz[N];

void Tarjan(int x)
{
	dfn[x] = low[x] = ++tot;
	stk[++top] = x; instk[x] = 1;
	for (int y : e[x])
	{
		if (!dfn[x])
		{
			Tarjan(y);
			low[x] = min(low[x], low[y]);
		}
		else if(instk[y])
		{
			low[x] = min(low[x], dfn[y]);
		}
	}

	if (low[x] == dfn[x])
	{
		int y; ++cnt;
		do
		{
			y = stk[top--];
			instk[y] = 0;
			scc[y] = cnt;
			siz[cnt]++;
		} while (x!=y);
	}
}

int main()
{
	int n, m;
	cin >> n >> m;
	for (int i = 1; i <= m; i++)
	{
		int u, v;
		cin >> u >> v;
		e[u].push_back(v);
	}

	for (int i = 1; i <= n; i++)
	{
		if (!dfn[i]) Tarjan(i);
	}

	for (int x = 1; x <= n; x++)
	{
		for (int y : e[x])
		{
			if (scc[x] != scc[y])
			{
				inn[scc[y]]++;
				outt[scc[x]]++;
			}
		}
	}

	int ans = 0,sum=0;

	for (int i = 1; i <= cnt; i++)
	{
		if (!outt[i])
		{
			sum++;
			ans += siz[i];
		}
	}
	if (sum == 1) cout << ans;
	else cout << 0;
	return 0;
}

如题,求大佬调教