Tarjan 割边(桥)

Posted 2020-12-02 water-radish

//Tarjan 割边 
//当dfn[u]<low[v]时u与v之间的边为割边 
#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<algorithm>
using namespace std;
int n,m,cnt,ans,head[10001];
int dot,dfn[10001],low[10001];
int cut[200001];
struct uio{
    int from,to,next,id;
}edge[200001];
void add(int x,int y)
{
    edge[++cnt].next=head[x];
    edge[cnt].from=x;
    edge[cnt].to=y;
    edge[cnt].id=cnt;
    head[x]=cnt;
}
void tarjan(int x,int fa)
{
    dfn[x]=low[x]=++dot;
    for(int i=head[x];i;i=edge[i].next)
    {
        int y=edge[i].to;
        if(!dfn[y])
        {
            tarjan(y,x);
            low[x]=min(low[x],low[y]);
            if(low[y]>dfn[x])
                 cut[edge[i].id]=1;
        }
        else if(y!=fa)
            low[x]=min(low[x],dfn[y]);
    }
}
int main()
{
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++)
    {
        int u,v;
        scanf("%d%d",&u,&v);
        add(u,v);
        add(v,u);
    }
    for(int i=1;i<=n;i++)
        if(!dfn[i])
            tarjan(i,i);
    for(int i=1;i<=cnt;i++)
        if(cut[i])
            ans++;
    printf("%d
",ans);
    for(int i=1;i<=cnt;i++)
        if(cut[i])
            printf("%d %d
",edge[i].from,edge[i].to);
     return 0;
}