poj3177双连通分量

#include<map>
#include<set>
#include<list>
#include<cmath>
#include<queue>
#include<stack>
#include<vector>
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define pii pair<int,int>
#define C 0.5772156649
#define pi acos(-1.0)
#define ll long long
#define mod 1000000007
#define ls l,m,rt<<1
#define rs m+1,r,rt<<1|1

using namespace std;
using namespace __gnu_cxx;

const double g=10.0,eps=1e-7;
const int N=5000+10,maxn=100000+10,inf=0x3f3f3f;

vector<int>v[N];
int dfn[N],low[N];
int index,num;
map<int,int>ma[N];
void tarjan(int u,int f)
{
    low[u]=dfn[u]=++index;
    for(int i=0;i<v[u].size();i++)
    {
        int x=v[u][i];
        if(x==f)continue;
        if(!dfn[x])
        {
            tarjan(x,u);
            low[u]=min(low[u],low[x]);
            if(low[x]>dfn[u])ma[x][u]=ma[u][x]=1;
        }
        else low[u]=min(low[u],dfn[x]);
    }
}
void dfs(int u,int f)
{
    dfn[u]=num;
    for(int i=0;i<v[u].size();i++)
    {
        int x=v[u][i];
        if(x!=u&&!dfn[x]&&!ma[x][u])dfs(x,u);
    }
}
void init(int n)
{
    memset(dfn,0,sizeof dfn);
    memset(low,0,sizeof low);
    index=num=0;
    for(int i=1;i<=n;i++)
    {
        v[i].clear();
        ma[i].clear();
    }
}
int main()
{
    int n,m;
    while(~scanf("%d%d",&n,&m))
    {
        init(n);
        for(int i=0; i<m; i++)
        {
            int a,b;
            scanf("%d%d",&a,&b);
            if(!ma[a][b])
            {
           //     cout<<a<<" "<<b<<endl;
                v[a].pb(b);
                v[b].pb(a);
                ma[a][b]=ma[b][a]=1;
            }
        }
        for(int i=1;i<=n;i++)ma[i].clear();
        for(int i=1; i<=n; i++)
            if(!dfn[i])
                tarjan(i,-1);
        memset(dfn,0,sizeof dfn);
        for(int i=1; i<=n; i++)
            if(!dfn[i])
            {
                ++num;
                dfs(i,-1);
            }
      //  cout<<num<<endl;
        memset(low,0,sizeof low);
        for(int i=1; i<=n; i++)
        {
            for(int j=0; j<v[i].size(); j++)
            {
                if(dfn[i]!=dfn[v[i][j]])
                {
                    low[dfn[i]]++;
                    low[dfn[v[i][j]]]++;
                }
            }
        }
        int ans=0;
        for(int i=1; i<=num; i++)
        {
            // cout<<low[i]<<endl;
            if(low[i]==2)
                ans++;
        }
        printf("%d\n",(ans+1)/2);
    }
    return 0;
}
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