#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;

const int N = 1e5, M = 1e5;
struct Edge {
    int v, next, idx;
    Edge(){}
    Edge(int _v, int _next, int _idx):
        v(_v), next(_next), idx(_idx){}
}e[M];
int low[N], dfn[N], deep, head[N], tot;
bool iscut[N], isbri[M];

void __init__()
{
    tot = deep = 0;
    memset(head, -1, sizeof(head));
    memset(iscut, 0, sizeof(iscut));
}

void add(int u, int v, int idx)
{
    e[tot++] = Edge(v, head[u], idx);
    head[u] = tot++;
}

int dfs(int u, int fa)
{
    int lowu = dfn[u] = ++deep;
    int cnt = 0;
    for(int i = head[u]; ~i; i = e[i].next) {
        int v = e[i].v;
        if(!dfn[v]) {
            cnt++;
            int lowv = dfs(v, u);
            lowu = min(lowu, lowv);
            if(lowv >= dfn[u]) iscut[u] = true;
            if(lowv > dfn[u]) isbri[e[i].idx] = true;
            continue ;
        }
        if(dfn[v] < dfn[u] && v != fa)
            lowu = min(lowu, dfn[v]);
    }
    if(fa == -1 && cnt == 1) iscut[u] = false;
    return low[u] = lowu;
}

int main()
{
    __init__();
    return 0;
}

【连通图】无向图关节点和桥 Tarjan