Bridges

#include <iostream>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <vector>
#include <tuple>
#include <map>
#include <set>
#include <queue>
#include <fstream>
using namespace std;
vector<vector<int>> g;
vector<int> tin, up;
vector<char> used;
map<pair<int, int>, int> section;
set<pair<int, int>> bridge;
int timer = 0;

void dfs(int v, int p){
    timer++;
    used[v] = true;
    tin[v] = up[v] = timer;
    for (auto to : g[v]){
        if (to == p) continue;
        if (used[to]){
            up[v] = min(up[v], tin[to]);
        } else {
            dfs(to, v);
            up[v] = min(up[v], up[to]);
            if (up[to] > tin[v]){
                bridge.insert({to, v});
                bridge.insert({v, to});
            }
        }
    }
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int n, m;
    cin >> n >> m;
    g.resize(n + 1);
    tin.resize(n + 1);
    up.resize(n + 1);
    used.resize(n + 1, false);
    map<pair<int, int>, bool> f;
    for (int i = 1; i <= m; i++){
        int a, b;
        cin >> a >> b;
        if (section.count({a, b}) || section.count({b, a})){
            f[{a, b}] = true;
            f[{b, a}] = true;
        } else if (a != b){
            section[{a, b}] = i;
            g[a].push_back(b);
            g[b].push_back(a);
        }
    }
    for (int i = 1; i <= n; i++){
        if (!used[i]) dfs(i, -1);
    }
    int summ = 0;
    vector<int> ans;
    for (auto i: section){
        if (bridge.count(i.first) && !f[i.first]){
            summ++;
            ans.push_back(i.second);
        }
    }
    cout << summ << "\n";
    for (auto i: ans){
        cout << i << " ";
    }
    return 0;
}