Tunnelit

#include <iostream>
#include <vector>
using namespace std;
#define N (1<<18)

int t[N], V, c[N];
vector<int> v[N], w;

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);
		++c[b];
	}
	for(int i=0; i<n; ++i) if(c[i+1]==0) w.push_back(i+1);
	
	for(unsigned int i=0; i<w.size(); ++i){
		int u = w[i];
		V+=max((int)v[u].size() - t[u], 0);
		for(int y : v[u]){
			++t[y];
			--c[y];
			if(c[y]==0) w.push_back(y);
		}
	}
	
	cout << V << "\n";
}

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