Tunnelit

#include <bits/stdc++.h>

#define ll long long

using namespace std;

vector<int> v[111111];
int out[111111];
int in[111111];
queue<int> q;

void dfs (int i) {
	if (v[i].empty()) return;
	int j = v[i].back();
	v[i].pop_back();
	in[j]--;
	out[i]--;
	if (!in[j]) q.push(j);
	dfs(j);
}

int main () {
	int n, m;
	cin>>n>>m;
	for (int i = 0; i < m; i++) {
		int a, b;
		cin>>a>>b;
		v[a].push_back(b);
		in[b]++;
		out[a]++;
	}
	int ans = 0;
	
	for (int i = 1; i <= n; i++) if (!in[i]) q.push(i);
	while (!q.empty()) {
		int x = q.front();
		q.pop();
		while (out[x]) {
			dfs(x);
			ans++;
		}
	}
	cout<<ans<<endl;
}

Test 1

Group: 1

Verdict: ACCEPTED

Test 2

Group: 1

Verdict: ACCEPTED

Test 3

Group: 1

Verdict: ACCEPTED

Test 4

Group: 1

Verdict: ACCEPTED

Test 5

Group: 1

Verdict: ACCEPTED

Test 6

Group: 2

Verdict: ACCEPTED

Test 7

Group: 2

Verdict: ACCEPTED

Test 8

Group: 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

Test 11

Group: 3

Verdict: ACCEPTED

Test 12

Group: 3

Verdict: ACCEPTED

Test 13

Group: 3

Verdict: ACCEPTED

Test 14

Group: 3

Verdict: ACCEPTED

Test 15

Group: 3

Verdict: ACCEPTED