题目来源：Light OJ 1291 Real Life Traffic

题意：最少添加几条边 可以使全图边双连通

思路：缩点 重新构图 答案就是（叶子节点数+1）/ 2

#include <vector>
#include <cstdio>
#include <cstring>
#include <stack>
#include <algorithm>
using namespace std;
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];
int dfs_clock;
int bcc_cnt;
int bri;
int n, m;
int degree[maxn];
stack <int> S;

void dfs(int u, int fa)
{
	low[u] = pre[u] = ++dfs_clock;
	S.push(u);
	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]);
		}
		else if(v != fa)
			low[u] = min(low[u], pre[v]);
	}
	if(low[u] == pre[u])
	{
		bcc_cnt++;
		while(1)
		{
			int x = S.top(); S.pop();
			bccno[x] = bcc_cnt;
			if(x == u)
				break;
		}
	}
}
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);
}
int main()
{
	int cas = 1;
	int T;
	scanf("%d", &T);
	while(T--)
	{
		scanf("%d %d", &n, &m);
		for(int i = 0; i <= n; i++)
			a[i].clear();
		while(m--)
		{
			int u, v;
			scanf("%d %d", &u, &v);
			a[u].push_back(v);
			a[v].push_back(u);
		}
		
		find_bcc();
		memset(degree, 0, sizeof(degree));
		for(int u = 0; u < n; u++)
		{
			for(int i = 0; i < a[u].size(); i++)
			{
				int v = a[u][i];
				if(bccno[u] != bccno[v])
				{
					degree[bccno[u]]++;
					degree[bccno[v]]++;
				}
			}
		}
		int ans = 0;
		for(int i = 1; i <= bcc_cnt; i++)
			if(degree[i] == 2)
				ans++;
		printf("Case %d: %d\n", cas++, (ans+1)/2);
	}
	return 0;
}