Tunnels

#include <bits/stdc++.h>

using namespace std;

#define rep(i, a, b) for(int i = a; i < int(b); ++i)
#define trav(a, v) for(auto& a : v)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()
#define D(x) cerr << #x << " = " << x << endl

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

int main() {
	int n, m;
	cin >> n >> m;
	vector < vector<bool> > activeE;
	vector < vector<int> > e;
	vector<int> eTo;
	activeE.resize(n);
	e.resize(n);
	eTo.resize(n);
	rep(i,0,n)
		eTo[i] = 0;
	set<int> finishedV;
	set<int> nextV;
	rep(i,0,m)
	{
		int tmp1, tmp2;
		cin >> tmp1 >> tmp2;
		activeE[tmp1 - 1].push_back(true);
		e[tmp1 - 1].push_back(tmp2 - 1);
		++eTo[tmp2 - 1];
	}
	rep(i,0,n)
	{
		if (eTo[i] == 0&&e[i].size()>0) {
			//cout<<eTo[i]<<" "<<e[i].size()<<" "<<i<<endl;
			nextV.insert(i);
		}
	}
	int days = 0;
	while (nextV.size() > 0) {
		int curr = (*nextV.begin());
		while (true) {
			int last = curr;
			int tunnels = 0;
			rep(i,0,e[last].size())
			{
				if (activeE[last][i]) {
					++tunnels;
					if (tunnels == 1) {
						activeE[curr][i] = false;
						curr = e[curr][i];
						--eTo[curr];
						if (eTo[curr] == 0) {
							nextV.insert(curr);
						}
					}
				}
			}
			if (tunnels == 1 || tunnels == 0) {
				nextV.erase(last);
			}
			if (tunnels == 0) {
				break;
			}
		}
		++days;
	}
	cout << days << 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