POJ 3177 Redundant Paths（边双连通分量）

#include <cstdio>
#include <algorithm>
#include <cstring>
#include <vector>
using namespace std;
const int N=5010,M=10010;
int e[M][2],cut[M],g[N],v[M<<1],nxt[M<<1],ed=1;
int f[N],dfn[N],low[N],num,cnt,from[N],d[N];
void add(int x,int y){v[++ed]=y;nxt[ed]=g[x];g[x]=ed;}
void tarjan(int x){
    dfn[x]=low[x]=++num;
    for(int i=g[x];i;i=nxt[i])if(!dfn[v[i]]){
        f[v[i]]=i>>1,tarjan(v[i]);
        if(low[x]>low[v[i]])low[x]=low[v[i]];
    }else if(f[x]!=(i>>1)&&low[x]>dfn[v[i]])low[x]=dfn[v[i]];
    if(f[x]&&low[x]==dfn[x])cut[f[x]]=1;
}
void dfs(int x,int y){
    from[x]=y;
    for(int i=g[x];i;i=nxt[i])if(!from[v[i]]&&!cut[i>>1])dfs(v[i],y);
}
int n,m; 
int main(){
    while(~scanf("%d%d",&n,&m)){
        memset(g,0,sizeof(g));
        memset(d,0,sizeof(d));
        memset(from,0,sizeof(from));
        memset(f,0,sizeof(f));
        memset(cut,0,sizeof(cut));
        num=0; ed=1; // g初始化为0的时候，ed一定要为1
        for(int i=1;i<=m;i++){
            int u,v;
            scanf("%d%d",&u,&v);
            e[i][0]=u; e[i][1]=v;
            add(u,v);add(v,u);
        }tarjan(1); cnt=0;
        for(int i=1;i<=n;i++)if(!from[i])dfs(i,++cnt);
        for(int i=1;i<=m;i++){
            if(from[e[i][0]]!=from[e[i][1]]){
                d[from[e[i][0]]]++;
                d[from[e[i][1]]]++;
            }
        }int res=0;
		    //for(int i=1;i<=n;i++)printf("%d %d\n",from[i],d[i]);
        for(int i=1;i<=n;i++)if(d[i]==1)res++;
        printf("%d\n",(res+1)/2);
    }return 0;
}