Tarjan:强连通分量 割点

有时间写 

Luogu P2341 [HAOI2006]受欢迎的牛 

#include<cstdio>
#include<iostream>
#define MogeKo qwq
using namespace std;

const int maxn = 100005;
int n,m,a,b,sum,ans;
int now,top,num;
int dfn[maxn],low[maxn],sta[maxn],out[maxn],col[maxn];
int cnt,to[maxn],nxt[maxn],head[maxn];
bool insta[maxn];

void add(int x,int y) {
    to[++cnt] = y;
    nxt[cnt] = head[x];
    head[x] = cnt;
}

void Tarjan(int u) {
    dfn[u] = low[u] = ++now;
    sta[++top] = u;
    insta[u] = true;
    for(int i = head[u]; i; i = nxt[i]) {
        int v = to[i];
        if(!dfn[v]) {
            Tarjan(v);
            low[u] = min(low[u],low[v]);
        } else if(insta[v])
            low[u] = min(low[u],dfn[v]);
    }
    if(low[u] == dfn[u]) {
        col[u] = ++num;
        insta[u] = false;
        while(sta[top]!=u) {
            int v = sta[top];
            col[v] = num;
            insta[v] = false;
            top--;
        }
        top--;
    }
}

int main() {
    scanf("%d%d",&n,&m);
    for(int i = 1; i <= m; i++) {
        scanf("%d%d",&a,&b);
        add(a,b);
    }
    for(int i = 1; i <= n; i++)
        if(!dfn[i])
            Tarjan(i);
    for(int u = 1; u <= n; u++)
        for(int i = head[u]; i; i = nxt[i]) {
            int v = to[i];
            if(col[u] != col[v])
                out[col[u]]++;
        }
    for(int i = 1; i <= num; i++) {
        if(!out[i]) {
            if(ans) {
                printf("0");
                return 0;
            }
            for(int u = 1; u <= n; u++)
                if(col[u] == i)ans++;
        }
    }
    printf("%d",ans);
    return 0;
}

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Luogu P3388 【模板】割点（割顶）

#include<cstdio>
#include<iostream>
#include<cstring>
#include<cmath>
#define MogeKo qwq
using namespace std;
const int maxn = 1e6+10;
const int INF = 2147483647;
int n,m,cnt,now,num,top,sum;
int x,y,z;
int to[maxn],head[maxn],nxt[maxn];
int dfn[maxn],low[maxn];
bool iscut[maxn];

void add(int x,int y) {
    to[++cnt] = y;
    nxt[cnt] = head[x];
    head[x] = cnt;
}

void Tarjan(int u,int fa) {
    dfn[u] = low[u] = ++now;
    int ch = 0;
    for(int i = head[u]; i; i = nxt[i]) {
        int v = to[i];
        if(!dfn[v]) {
            if(fa == u)ch++;
            Tarjan(v,fa);
            low[u] = min(low[u],low[v]);
            if(fa != u && low[v] >= dfn[u]) iscut[u] = true;
        }
        low[u] = min(low[u],dfn[v]);
        if(fa == u && ch >= 2) iscut[u] = true;
    }
}

int main() {
    scanf("%d%d",&n,&m);
    for(int i = 1; i <= m; i++) {
        scanf("%d%d",&x,&y);
        add(x,y);
        add(y,x);
    }
    for(int i = 1; i <= n; i++)
        if(!dfn[i])
            Tarjan(i,i);
    for(int i = 1; i <= n; i++)
        if(iscut[i])sum++;
    printf("%d\n",sum);
    for(int i = 1; i <= n; i++)
        if(iscut[i]) printf("%d ",i);
    return 0;
}

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