tarjan

强连通分量

int low[maxn],dfn[maxn];
int sta[maxn];//secondary stack in SCC
int point[maxn];//scc that vertices belong to
bool omit[maxn];//if the vertiex have been visited and poped from stack, omit=1
int top,times,tot;
void scc(int now)//strongly connectly component
{
	times++;
	dfn[now]=low[now]=times;
	top++;
	sta[top]=now;
	omit[now]=0;
	for (auto to:A[now])
	{
		if (!dfn[to])//have not been visited
		{
			scc(to);
			low[now]=min(low[now],low[to]);//update low,maybe our son have discovered a whole new world
		}
		else if (!omit[to])
		{
			low[now]=min(low[now],dfn[to]);//yes i have discovered a whole new world
		}
	}
	if (low[now]==dfn[now])//finally,i came full circle
	{
		tot++;
		do
		{		
//			totalval[tot]+=val[sta[top]];
			point[sta[top]]=tot; 
			omit[sta[top]]=1;
			top--;
		}
		while (sta[top+1]!=now);
	}
}
//init FOR(i,1,2*n+5) low[i] = dfn[i] = omit[i] = sta[i] = point[i] = 0;

双联通分量

#include <bits/stdc++.h>
#pragma GCC diagnostic error "-std=c++11"
#define SIS std::ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
//#define endl '
'
#define all(A) (A).begin(),(A).end()
#define FOR(I, A, B) for (int I = (A); I <= (B); ++I)
#define PER(I, A, B) for (int I = (A); I >= (B); --I)
#define lson k*2
#define rson k*2+1
#define fi first
#define se second
#define DB(A) cout<<(A)<<endl
#define DB1(A,B,C) cout<<(A)<<" "<<(B)<<" "<<(C)<<"!"<<endl
#define PB push_back
#define Pair pair<int,int>
#define MP make_pair
#define LL long long
using namespace std;
const int maxn=1000+10;
const int MAX=1000;
const int inf=0x3f3f3f3f;   
const int mod=1e9+7;
//head
int n,m;

int low[maxn],dfn[maxn];
int sta[maxn];
int t,times,top,tot;
int isbridge[maxn];

int ne[maxn],u[maxn],v[maxn],fi[maxn];//forward_star
void edge(int x,int y)
{
	t++;
	ne[t]=fi[x];
	fi[x]=t;
	u[t]=x;
	v[t]=y;
}

void bcc(int now,int fa)//biconnected component，求割点 
{
	times++;
	dfn[now]=low[now]=times;
//	top++;
//	sta[top]=now;
//	omit[now]=0;
	for (int i=fi[now];i!=-1;i=ne[i])
	{
		if (!dfn[v[i]])//have not been visited
		{
			bcc(v[i],now);
			if (low[v[i]]<low[now]) low[now]=low[v[i]];
			if (dfn[now]>low[v[i]]) isbridge[i]=0,isbridge[i^1]=0;
			//low[now]=min(low[now],low[v[i]]);//update low,maybe our son have discovered a whole new world
		}
		else if (v[i]!=fa)//if (!omit[v[i]])
		{
			if (dfn[v[i]]<low[now]) low[now]=dfn[v[i]];//yes i have discovered a whole new world
		}
	}
//	if (low[now]==dfn[now])//finally,i came full circle
//	{
//		tot++;
//		do
//		{		
//			dis[tot]+=d[sta[top]];
//			point[sta[top]]=tot; 
//			omit[sta[top]]=1;
//			top--;
//		}
//		while (sta[top+1]!=now);
//	}
}
int main()
{

	scanf("%d%d",&n,&m);
	
	t=-1;
	top=0;
	tot=0;
	times=0;
//	memset(dis,0,sizeof(dis));	
//	memset(dfn,0,sizeof(dfn));
//	memset(num,0,sizeof(num));
	for (int i=1;i<=n;i++) fi[i]=-1;
	for (int i=1;i<=m;i++)
	{
		int x,y;
		scanf("%d%d",&x,&y);
		edge(x,y);
		edge(y,x);
	}
	for (int i=1;i<=m*2;i++) isbridge[i]=1;
	for (int i=1;i<=n;i++) 
		if (!dfn[i]) bcc(i,0);		

//	FOR(i,0,m*2-1) if (isbridge[i]==1) return DB(0),0;
//	dfs(1);
	return 0;
}