【模板】图的割点

lrj
dfn：发现时间
low：子树中可以追溯到最早的节点的发现时间

#include<iostream>#include<cstdio>#include<cstring>#include<algorithm>using namespace std;const int M=501000;const int N=101000;struct E {int to,nxt;}edge[M*2];int low[N],dfn[N],Time=0;int idx[N],tot=1;bool iscut[N];int n,m,ans;void addedge(int from,int to){    edge[tot].to=to;edge[tot].nxt=idx[from];idx[from]=tot++;}void dfs(int x,int fa){    int child=0;    dfn[x]=low[x]=++Time;    for(int t=idx[x];t;t=edge[t].nxt){        E e=edge[t];        if(!dfn[e.to]){            child++;            dfs(e.to,x);            low[x]=min(low[e.to],low[x]);            if(low[e.to]>=dfn[x]) iscut[x]=1;        }        else if(e.to!=fa)            low[x]=min(low[x],dfn[e.to]);    }    if(fa==-1 && child==1) iscut[x]=0;}int main(){//  freopen("in.txt","r",stdin);//  freopen("out.txt","w",stdout);    scanf("%d%d",&n,&m);    for(int i=1;i<=m;i++){        int x,y;        scanf("%d%d",&x,&y);        addedge(x,y);addedge(y,x);    }    dfs(1,-1);    for(int i=1;i<=n;i++) if(iscut[i]) ans++;    printf("%d\n",ans);    return 0;}