03-algorithms_on_graphs/02-decomposition/01-acyclicity/acyclicity.cpp


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#include <iostream>
#include <vector>
#include <stack>

using std::vector;
using std::stack;

int acyclic(vector<vector<int> > &adj) {
    vector<bool> marked(adj.size());
    vector<bool> visited(adj.size());

    stack<int> st;

    for (int i = 0; i < adj.size(); i++) {
        if (marked[i])
            continue;
        st.push(i);
        while(!st.empty()) {
            int v = st.top();
            visited[v] = true;
            int s = adj[v].size();
            for (int j = 0; j < s; j++) {
                int w = adj[v][j];
                if (!marked[w]) {
                    if (visited[w])
                        return 1;
                    st.push(w);
                }
            }

            if (st.top() == v) {
                visited[v] = false;
                marked[v] = true;
                st.pop();
            }
        }
    }

    return 0;
}

int main() {
  size_t n, m;
  std::cin >> n >> m;
  vector<vector<int> > adj(n, vector<int>());
  for (size_t i = 0; i < m; i++) {
    int x, y;
    std::cin >> x >> y;
    adj[x - 1].push_back(y - 1);
  }
  std::cout << acyclic(adj) << '\n';
}