求割点和点双连通分量

const int maxn = 1010;
vector <int> a[maxn], bcc[maxn];
int pre[maxn];
int low[maxn];
bool iscut[maxn];
int bccno[maxn];
int cnt[maxn];
int dfs_clock;
int bcc_cnt;
int n;

struct Edge
{
	int u, v;
};

stack <Edge> S;
int dfs(int u, int fa)
{
	int lowu = pre[u] = ++dfs_clock;
	int child = 0;
	for(int i = 0; i < a[u].size(); i++)
	{
		int v = a[u][i];
		Edge e = (Edge){u, v};
		if(!pre[v])
		{
			S.push(e);
			child++;
			int lowv = dfs(v, u);
			lowu = min(lowu, lowv);
			if(lowv >= pre[u])
			{
				iscut[u] = true;
				bcc_cnt++;
				bcc[bcc_cnt].clear();
				cnt[u]++;
				while(1)
				{
					Edge x = S.top();
					S.pop();
					if(bccno[x.u] != bcc_cnt)
					{
						bcc[bcc_cnt].push_back(x.u);
						bccno[x.u] = bcc_cnt;
					}
					if(bccno[x.v] != bcc_cnt)
					{
						bcc[bcc_cnt].push_back(x.v);
						bccno[x.v] = bcc_cnt;
					}
					if(x.u == u && x.v == v)
						break;
				}
			}
		}
		else if(pre[v] < pre[u] && v != fa)
		{
			S.push(e);
			lowu = min(lowu, pre[v]);
		}
	}
	if(fa < 0 && child == 1)
	{
		iscut[u] = false;
		cnt[u] = 0;
	}
	if(fa < 0 && child > 1)
	{
		iscut[u] = true;
		cnt[u] = child;
	}
	else if(cnt[u] > 0)
		cnt[u]++;
	return lowu;
}
void find_bcc()
{
	memset(cnt, 0, sizeof(cnt));
	memset(pre, 0, sizeof(pre));
	memset(iscut, 0, sizeof(iscut));
	memset(bccno, 0, sizeof(bccno));
	dfs_clock = bcc_cnt = 0;
	dfs(1, -1);
}

求桥和双连通

#include <cstdio>
#include <cstring>
#include <vector>
#include <stack>
#include <algorithm>
using namespace std;
const int maxn = 5010;

vector <int> G[maxn];
bool ok[maxn][maxn];
int pre[maxn];
int low[maxn];
int sccno[maxn];
int dfs_clock;
int scc_cnt;
stack <int> S;
int n, m;
int degree[maxn];
int Topo[maxn][maxn];
void dfs(int u, int fa)
{
	pre[u] = low[u] = ++dfs_clock;
	S.push(u);
	for(int i = 0; i < G[u].size(); i++)
	{
		int v = G[u][i];
		if(v == fa)
			continue;
		if(!pre[v])
		{
			dfs(v, u);
			low[u] = min(low[u], low[v]);
		}
		else
			low[u] = min(low[u], pre[v]);
	}
	if(low[u] == pre[u])
	{
		scc_cnt++;
		while(1)
		{
			int x = S.top();
			S.pop();
			sccno[x] = scc_cnt;
			if(x == u)
				break;
		}
	}
}
void find_scc()
{
	dfs_clock = scc_cnt = 0;
	memset(sccno, 0, sizeof(sccno));
	memset(pre, 0, sizeof(pre));
	for(int i = 1; i <= n; i++)
		if(!pre[i])
			dfs(i, -1);
}

const int maxn = 10010;
struct Edge
{
	int u, v;
	Edge(){}
	Edge(int u, int v): u(u), v(v){}
};
vector <int> a[maxn];
int pre[maxn];
int low[maxn];
int bccno[maxn];
Edge cnt[maxn];
int dfs_clock;
int bcc_cnt;
int bri;
int n, m;

stack <Edge> S;
bool cmp(Edge a, Edge b)
{
	if(a.u != b.u)
		return a.u < b.u;
	return a.v < b.v;
}
void dfs(int u, int fa)
{
	low[u] = pre[u] = ++dfs_clock;
	for(int i = 0; i < a[u].size(); i++)
	{
		int v = a[u][i];
		//if(v == fa)
		//	continue;
		if(!pre[v])
		{
			dfs(v, u);
			low[u] = min(low[u], low[v]);
			if(low[v] > pre[u])
			{
				if(u < v)
				{
					cnt[bri].u = u;
					cnt[bri].v = v;
				}
				else
				{
					cnt[bri].u = v;
					cnt[bri].v = u;
				}
				bri++;
			}
		}
		else if(v != fa)
			low[u] = min(low[u], pre[v]);
	}
}
void find_bcc()
{
	memset(pre, 0, sizeof(pre));
	memset(bccno, 0, sizeof(bccno));
	dfs_clock = bcc_cnt = bri = 0;
	for(int i = 0; i < n; i++)
		if(!pre[i])
			dfs(i, -1);
}