bridges

vector<vector<pair<int, int>>> g;
vector<int> used;
vector<int> up;
vector<int> tin;
vector<int> ans;
int timer = 0;

void dfs(int v, int parent, int parent_edge) {
    tin[v] = up[v] = timer++;
    used[v] = 1;
    for (auto[u, edge] : g[v]) {
        if (u == parent && edge == parent_edge) {
            continue;
        }
        if (used[u]) {
            up[v] = min(up[v], tin[u]);
        } else {
            dfs(u, v, edge);
            up[v] = min(up[v], up[u]);
            if (up[u] > tin[v]) {
                ans.push_back(edge);
            }
        }
    }
}