Tunnelit

#include <iostream>
#include <vector>

const int N = 1e5;
int ins [N];
std::vector<int> graph [N];

int main() {
	std::ios_base::sync_with_stdio(false);
	std::cin.tie(0);

	int n, m;
	std::cin >> n >> m;
	for (int i = 0; i < n; ++i) {
		ins[i] = 0;
		std::vector<int> vec;
		graph[i] = vec;
	}
	int a, b;
	for (int i = 0; i < m; ++i) {
		std::cin >> a >> b;
		--a;
		--b;
		graph[a].push_back(b);
		++ins[b];
	}
	std::vector<int> handle;
	for (int i = 0; i < n; ++i) {
		if (ins[i] == 0) {
			handle.push_back(i);
		}
	}
	int ind = 0;
	int days = 0;
	while(ind < (int)handle.size()) {
		int i = handle[ind];
		if (graph[i].empty()) {
			++ind;
			continue;
		}
		++days;
		while(! (graph[i].empty())) {
			int next = graph[i][graph[i].size() - 1];
			--ins[next];
			if (ins[next] == 0) {
				handle.push_back(next);
			}
			graph[i].pop_back();
			i = next;
		}	
	}
	std::cout << days << "\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