Tunnels

#include <bits/stdc++.h>
//#define cin in
typedef long long ll;

using namespace std;

const int maxA = 100000 + 5;

int a, b;
queue<int> from[maxA];
int to[maxA];


int main()
{
    ifstream in("test.txt");
    cin >> a >> b;
    int t1, t2;
    for (int i=0; i<b; i++) {
        cin >> t1 >> t2;
        from[t1].push(t2);
        to[t2]++;
    }
    int ats=0;
    queue <int> ends;
    for (int i=1; i<=a; i++) {
        if (to[i]==0 && !from[i].empty()) {
            ends.push(i);
            ats++;
        }
    }

    while (!ends.empty()) {
        int curr = ends.front();
        //cout << curr << ": ";
        ends.pop();
        if (from[curr].size()>1) {
                ats++;
                ends.push(curr);
        }

        while (!from[curr].empty()) {
            int sec = from[curr].front();
            from[curr].pop();
            to[sec]--;
            //cout << sec << " ";
            if (to[sec]==0 && from[sec].size()>1) {
                ats++;
                ends.push(sec);
            }
            curr=sec;
        }
        //cout << endl;
    }
    cout << ats;

    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