hdu 4738 Caocao's Bridges（双联通）

一个无向图  炸一个桥  使无向图分成两个以上部分 问所炸桥的最小权值  。

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<queue>
#include<map>
using namespace std;
#define ma_pa(a,b) make_pair(a,b)
#define clr(x) memset(x,0,sizeof(x))
#define maxn 2010
#define maxm 2100000
#define INF 0x1f1f1f1f
struct node{
    int to,next;
}e[maxm],e2[maxm];
int head[maxn],tot;
int head2[maxn],tot2;
int col[maxn],dfn[maxn],low[maxn],sta[maxn];
int top,ti,scc;  //scc 双联通分支个数
int n,m;
bool vis[maxn];
void init(){
    memset(head,-1,sizeof(head));
    clr(vis);
    clr(dfn);
    tot=0;
    top=0;
    scc=ti=0;
}
void add(int u,int v){
    e[tot].to=v;
    e[tot].next=head[u];
    head[u]=tot++;
}
void tarjan(int u,int fa){
    dfn[u]=low[u]=++ti;
    sta[++top]=u;
    vis[u]=true;
    int cnt=0;
    for(int i=head[u];i!=-1;i=e[i].next){
        int v=e[i].to;
        if(!dfn[v]){
            tarjan(v,u);
            low[u]=min(low[u],low[v]);
        }
        else if(fa==v){
            if (cnt) low[u] = min(low[u],dfn[v]);//重边
            cnt++;
        }
        else if(vis[v])low[u]=min(low[u],dfn[v]);
    }
    if(dfn[u]==low[u]){
        int x;
        scc++;
        do{
            x = sta[top--];
            vis[x]=false;
            col[x] = scc;
        }while(x!=u);
    }
}
int father[4010];
int find(int x)
{
    if(father[x]==x)return x;
    return father[x]=find(father[x]);
}
bool join(int x,int y)
{
    x=find(x);
    y=find(y);
    if(x==y)return false;
    if(x>y){father[x]=y;}
    if(x<y){father[y]=x;}
    return true;
}
struct node1{
    int x,y;
}road[2100000];
int main(){
    int u,v,w,i;
    while(scanf("%d%d",&n,&m)!=EOF){
        init();
        map<pair<int,int>,int>mp;
        mp.clear();
        for(i=0;i<=n;i++){
            father[i]=i;
        }
        if(n==0&&m==0)break;
        int ppp=INF;
        for(int i=0;i<m;i++){
            scanf("%d%d%d",&u,&v,&w);
            if(v==u)continue;
            add(u,v);
            add(v,u);
            join(u,v);
            ppp=min(ppp,w);
            if(mp[ma_pa(u,v)]==0){
                mp[ma_pa(u,v)]=w;
                mp[ma_pa(v,u)]=w;
            }
            else {
                mp[ma_pa(u,v)]=min(mp[ma_pa(u,v)],w);
                mp[ma_pa(v,u)]=mp[ma_pa(u,v)];
            }

        }
        int fff=0;
        for(i=1;i<=n;i++){
            if(find(i)!=1)fff=1;
        }
        if(fff){printf("0\n");continue;}
        tarjan(1,-1);
        if(scc==0){
            printf("%d\n",ppp);
            continue;
        }
        if(scc==1){
            printf("-1\n");
            continue;
        }
        int res=INF;
        for(int i = 1; i <= n; i++){
            for(int j = head[i]; j!=-1; j = e[j].next){
                int v = e[j].to;
                if(col[i]!=col[v]){
                    res=min(res,mp[ma_pa(i,v)]);
                }
            }
        }
        if(res==0)printf("1\n");
        else printf("%d\n",res);
    }
    return 0;
}