割点代码

还是有些细节要注意的。。

#include <cstdio>
using namespace std;

const int maxn=2e5+5;

inline int min(int a, int b) { return a<b?a:b; }

class Graph{
public:
    struct Edge{
        int to, next; Graph *belong;
        Edge& operator ++(){ *this=belong->edge[next]; return *this; }
        int operator *(){ return to; }
    };
    void addedge(int u, int v){
        Edge &e=edge[++cntedge];
        e.to=v, e.next=fir[u];
        e.belong=this, fir[u]=cntedge;
    }
    Edge gete(int x) { return edge[fir[x]]; }
private:
    int cntedge, fir[maxn];
    Edge edge[maxn];
};

int n, m, time, cntgd;
int dfn[maxn], low[maxn], gd[maxn];
Graph g;

//无向图求割点，要么点未访问，要么已退栈，不可能在栈中。
void tarjan(int now, int par){
    dfn[now]=low[now]=++time;
    int nowson, child=0;
    for (Graph::Edge e=g.gete(now); *e; ++e){
        nowson=*e;
        if (nowson==par) continue;
        if (!dfn[nowson]){
            //注意child是递归树中的孩子。。
            ++child;
            tarjan(nowson, now);
            low[now]=min(low[now], low[nowson]);
            //注意只要有一个分支这样就是割点！
            if (low[nowson]>=dfn[now]) gd[now]=1;
        } else low[now]=min(low[now], dfn[nowson]);
    }
    //根节点肯定为1呀，应该赋值成0！
    if (!par&&child==1) gd[now]=0;
    if (gd[now]) ++cntgd;
}

int main(){
    scanf("%d%d", &n, &m);
    int x, y;
    for (int i=0; i<m; ++i){
        scanf("%d%d", &x, &y);
        g.addedge(x, y); g.addedge(y, x);
    }
    for (int i=1; i<=n; ++i)
        if (!dfn[i]) tarjan(i, 0);
    printf("%d
", cntgd);
    for (int i=1; i<=n; ++i)
        if (gd[i]) printf("%d ", i);
    return 0;
}