Programación Competitiva

/*
4 4
1 2
3 2
4 3
2 1
Output:

2
1 2
*/

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


    using namespace std;


    const int MAX = 100005;


    int N, M;
    int componentCount[2];
    int componentID[2][MAX];
    int degree[MAX];
    bool visited[2][MAX];
    vector<int> orders;
    vector<int> Graph[MAX], reverseGraph[MAX];


    void dfs1 (int node) {
        visited[0][node] = true;
        componentID[0][node] = componentCount[0];
        for (int i : reverseGraph[node]) {
            if (!visited[0][i]) {
                dfs1(i);
            }
        }
        orders.push_back(node);
    }
    void dfs2 (int node) {
        visited[1][node] = true;
        componentID[1][node] = componentCount[1];
        for (int i : Graph[node]) {
            if (!visited[1][i]) {
                dfs2(i);
            }
        }
    }


    int main(int argc, const char * argv[]) {
        scanf("%d%d", &N, &M);
        for (int i = 0; i < M; i ++) {
            int u, v;
            scanf("%d%d", &u, &v);
            Graph[u].push_back(v);
            reverseGraph[v].push_back(u);
        }
        for (int i = 1; i <= N; i ++) {
            if (!visited[0][i]) {
                componentCount[0] ++;
                dfs1(i);
            }
        }
        reverse(orders.begin(), orders.end());
        for (int i : orders) {
            if (!visited[1][i]) {
                componentCount[1] ++;
                dfs2(i);
            }
        }
        //for (int i = 1; i <= N; i ++) {
        //    cout << i << " --> " << componentID[1][i] << endl;
        //}
        //cout << endl;
        for (int i = 1; i <= N; i ++) {
            for (int j : reverseGraph[i]) {
                if (componentID[1][i] != componentID[1][j]) {
                    degree[componentID[1][j]] ++;
                }
            }
        }
        //for (int i = 1; i <= componentCount[1]; i ++) {
        //    cout << i << " --> " << degree[i] << endl;
        //}
        int startings = 0;
        for (int i = 1; i <= componentCount[1]; i ++) {
            if (!degree[i]) {
                startings ++;
            }
        }
        if (startings > 1) {
            puts("0");
        }
        else {
            vector<int> output;
            for (int i = 1; i <= N; i ++) {
                if (!degree[componentID[1][i]]) {
                    output.push_back(i);
                }
            }
            printf("%d\n", (int)output.size());
            for (int i = 0; i < (int)output.size(); i ++) {
                printf("%d", output[i]);
                if (i < (int)output.size() - 1) {
                    putchar(' ');
                }
            }
            putchar('\n');
        }

        return 0;
    }