Tunnelit

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

using namespace std;

int n, m;
unordered_set<int> g[100005];
vector<int> ts;
int v[100005];

int d = 0;
int st;

void topos(int s) {
    if (v[s]) return;
    v[s] = 1;
    for (int a : g[s]) {
        topos(a);
    }
    ts.push_back(s);
}

void es(int s) {
    if (g[s].size() != 0) {
        int nn = *(g[s].begin());
        es(nn);
        g[s].erase(nn);
    }
}

int main() {
    cin >> n >> m;
    for (int i = 0; i < m; ++i) {
        int a, b;
        cin >> a >> b;
        g[a].insert(b);
    }
    for (int i = 1; i <= n; ++i) topos(i);
    reverse(ts.begin(), ts.end());
    st = 0;
    
    while (true) {
        while (g[ts[st]].size() == 0) {
            st++;
            if (st > n) {
                cout << d << "\n";
                return 0;
            }
        }
        d++;
        es(ts[st]);
    }
    
    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