Tarjan 割点 割边

#include<algorithm>
#include<cstdio>
#include<vector>
#define maxn 20000
#define maxm 120
using namespace std;
vector<int> e[maxn];
int gd[maxm];
int dfn[maxm],low[maxm];
int n,root,dtime,ans;
void tarjan(int u){
    int i,v,tot=0;
    low[u]=dfn[u]=++dtime;
    for(i=0;i<e[u].size();i++){
        v=e[u][i];
        if(!dfn[v]){
            tarjan(v);
            ++tot;
            low[u]=min(low[v],low[u]);
            if((u==root&&tot>1)||(u!=root&&low[v]>=dfn[u]))
                if(!gd[u]) gd[u]=true,ans++;
        }
        else low[u]=min(low[u],dfn[v]);
    }
    return ;
}
int main(){
    scanf("%d",&n);
    int i,j,a,b,u,v;
    while(scanf("%d%d",&u,&v)!=EOF){
        e[u].push_back(v);
        e[v].push_back(u);
    }
    for(i=1;i<=n;i++){
        if(!dfn[i]){
            root=i;
            tarjan(i);
        }
    }
    printf("%d\n",ans);
    for(i=1;i<=n;i++){
        if(gd[i])
            printf("%d\n",i);
    }
    return 0;
}

#include<cstdio>
#include<vector>
#include<cstring>
#include<algorithm>
#define maxn 155
#define maxm 5005
using namespace std;
vector<int> vec[maxn];
bool bridge[maxn],vis[maxn];
int low[maxn],dfn[maxn];
int head[maxn],pre[maxn],ip,sol,tot;
int n,m,u,v,dtime;
struct node{
    int v,next,use;
}edge[maxm<<2];
struct Bridge{
    int u,v;
}bri[maxm];
bool cmp(Bridge x,Bridge y){
    if(x.u!=y.u)
        return x.u<y.u;
    else
        return x.v<y.v;
}
void tarjan(int u){
    vis[u]=1;
    dfn[u]=low[u]=dtime++;
    for(int i=head[u];i!=-1;i=edge[i].next){
        if(!edge[i].use){
            edge[i].use=edge[i^1].use=1;
            int v=edge[i].v;
            if(!vis[v]){
                pre[v]=u;
                tarjan(v);
                low[u]=min(low[u],low[v]);
                if(dfn[u]<low[v]){
                    sol++;
                    bridge[v]=1;
                }
            }
            else{
                low[u]=min(low[u],dfn[v]);
            }
        }
    }
}
int main(){
    memset(head,-1,sizeof(head));
    scanf("%d%d",&n,&m);
    for(int i=0;i<m;i++){
        scanf("%d%d",&u,&v);
        edge[ip].v=v;
        edge[ip].use=0;
        edge[ip].next=head[u];
        head[u]=ip++;
        edge[ip].v=u;
        edge[ip].use=0;
        edge[ip].next=head[v];
        head[v]=ip++;
    }
    pre[1]=1;
    tarjan(1);
    for(int i=1;i<=n;i++){
        if(bridge[i]){
            tot++;
            if(pre[i]>i){
                bri[tot].u=i;
                bri[tot].v=pre[i];
            }
            else{
                bri[tot].u=pre[i];
                bri[tot].v=i;
            }
        }
    }
    sort(bri+1,bri+tot+1,cmp);
    for(int i=1;i<=tot;i++)
        printf("%d %d\n",bri[i].u,bri[i].v);
    return 0;
}

读入的都是无向图~