Tunnels

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

typedef struct
{
	vector<int> nextRoom, previousRoom;
} Room;

int n, m;
Room room[100000];

int main() {
	cin >> n >> m;

	int a, b;
	for (int i = 0; i < m; i++)
	{
		cin >> a >> b;
		a--;
		b--;
		room[a].nextRoom.push_back(b);
		room[b].previousRoom.push_back(a);
	}

	int d, i = 0, t;
	for (d = 0; m > 0; d++)
	{
		while (room[i].nextRoom.size() == 0 && room[i].previousRoom.size() == 0)
			i++;
		a = i;
		b = i;
		while (room[a].nextRoom.size() != 0)
		{
			t = room[a].nextRoom.back();
			room[a].nextRoom.pop_back();
			room[t].previousRoom.erase(std::remove(room[t].previousRoom.begin(), room[t].previousRoom.end(), a), 
				room[t].previousRoom.end());
			a = t;
			m--;
		}
		while (room[b].previousRoom.size() != 0)
		{
			t = room[b].previousRoom.back();
			room[b].previousRoom.pop_back();
			room[t].nextRoom.erase(std::remove(room[t].nextRoom.begin(), room[t].nextRoom.end(), b),
				room[t].nextRoom.end());
			b = t;
			m--;
		}
	}

	cout << d << endl;

	/*cout << endl << "Press enter to continue...";
	cin.ignore(numeric_limits<streamsize>::max(), '\n');
	cin.get();*/

	return 0;
}

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