codeforces 962 F Simple Cycles Edges

求简单环，即求点=边数的点双分量，加上判断点和边的模板即可 (简单环模板，区分与点双缩点)

const int maxm = 1e5+5;

int head[maxm<<1], edgecnt, dfn[maxm], low[maxm], bcc_cnt, bccnum[maxm], dfs_clock, s[maxm], top;
vector<int> bcc[maxm], ans;

struct edge{
    int u, v, nex;
} edges[maxm<<1];

void addedge(int u, int v) {
    edges[++edgecnt].u = u;
    edges[edgecnt].v = v;
    edges[edgecnt].nex = head[u];
    head[u] = edgecnt;
}

void tarjan(int u, int fa) {
    dfn[u] = low[u] = ++dfs_clock;
    int child = 0, v;
    for(int i = head[u]; i; i = edges[i].nex) {
        v = edges[i].v;
        if(v == fa) continue;
        if(!dfn[v]) {
            s[++top] = i;
            tarjan(v, u);
            low[u] = min(low[u], low[v]);
            if(low[v] >= dfn[u]) {
                bcc[++bcc_cnt].clear();
                int vet = 0;
                bool ok = true;
                while(true) {
                    int num = s[top--];
                    bcc[bcc_cnt].push_back((num+1)/2);                                      
                    if(bccnum[edges[num].v] != bcc_cnt) {
                        vet++;
                        bccnum[edges[num].v]=bcc_cnt;
                    } else ok = false;
                    if(edges[num].u == u && edges[num].v == v) break;
                }
                if(ok && vet>1)
                    for(int i = 0; i < bcc[bcc_cnt].size(); ++i)
                        ans.push_back(bcc[bcc_cnt][i]);
            }
        } else if(dfn[v] < dfn[u]){
            s[++top] = i;
            low[u] = min(low[u], dfn[v]);
        }
    }
}

void run_case() {
    int n, m, u, v;
    cin >> n >> m;
    for(int i = 1; i <= m; ++i) {
        cin >> u >> v;
        addedge(u, v), addedge(v, u);
    }
    for(int i = 1; i <= n; ++i)
        if(!dfn[i])
            tarjan(i, -1);
    sort(ans.begin(), ans.end());
    cout << ans.size() << "
";
    for(int i : ans)
        cout << i << " ";
    
}

int main() {
    ios::sync_with_stdio(false), cin.tie(0);
    run_case();
    return 0;
}