hdu 3849 (雙聯通求橋)

#include <iostream>
#include <cstring>
#include <cstdio>
#include <map>
#include <string>
#include <algorithm>
#define N 10001
using namespace std;
int head[N],num,dfs[N],low[N],n,m,idx,bridge[N],bum;
struct edge
{
	int st,ed,next;
}E[N*20];
void addedge(int x,int y)
{
	E[num].st=x;
	E[num].ed=y;
	E[num].next=head[x];
	head[x]=num++;
}
int cmp(const void *a,const void *b)
{
    return *(int *)a-*(int *)b;
}
void Tarjan(int u,int father)
{
    int i,v;
    low[u]=dfs[u]=idx++;
    for(i=head[u];i!=-1;i=E[i].next)
    {
        v=E[i].ed;
        if(v==father)continue;
        if(dfs[v]==-1)
        {
            Tarjan(v,u);
            low[u]=low[u]>low[v]?low[v]:low[u];
            if(low[v]>dfs[u])//橋
            {
				bridge[bum++]=i;
            }
        }
        else low[u]=low[u]>dfs[v]?dfs[v]:low[u];
    }
}
int main()
{
	int i,x,y,j,t,sum;
	char str[2][200];
	scanf("%d",&t);
	while(t--)
    {
        scanf("%d%d",&n,&m);
		memset(head,-1,sizeof(head));
		num=0;sum=1;
		map<string,int>mp;
        map<int,string>mmp;
		for(i=0;i<m;i++)
		{
			scanf("%s%s",str[0],str[1]);
			if(mp[str[0]]==0)
			{
				mp[str[0]]=sum;
				mmp[sum]=str[0];
				sum++;
			}
			if(mp[str[1]]==0)
			{
				mp[str[1]]=sum;
				mmp[sum]=str[1];
				sum++;
			}
			x=mp[str[0]];
			y=mp[str[1]];
			addedge(x,y);
			addedge(y,x);
		}
		memset(dfs,-1,sizeof(dfs));
		idx=0;
		bum=0;
		Tarjan(1,-1);
		for(i=1;i<=n;i++)
        {
            if(dfs[i]==-1)
                break;
        }
        if(i<=n){printf("0\n");continue;}
		printf("%d\n",bum);
		qsort(bridge,bum,sizeof(bridge[0]),cmp);
		for(j=0;j<bum;j++)
        {
            i=bridge[j];
            i=(i/2*2);
            x=E[i].st;
            y=E[i].ed;
            if(x!=y)
                printf("%s %s\n",mmp[E[i].st].c_str(),mmp[E[i].ed].c_str());
        }
	}
	return 0;
}