HDU 2121 Ice_cream’s world II 最小树形图

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <string>
#include <stack>
#include <queue>
#include <cmath>
#include <vector>
using namespace std;
typedef long long LL;
const int N=2e3+5;
const LL INF=0x7fffffff;
struct Edge{
    int u,v;
    LL w;
}edge[N*N];
LL in[N];
int id[N],vis[N],pre[N],n,m,pos;
LL zhuliu(int rt,int n,int m){
     LL ret=0;
     while(1){
     for(int i=0;i<=n;++i)in[i]=INF;
     for(int i=1;i<=m;++i){
        if(edge[i].u!=edge[i].v&&edge[i].w<in[edge[i].v]){
            if(edge[i].u==rt)pos=i;
            pre[edge[i].v]=edge[i].u;
            in[edge[i].v]=edge[i].w;
        }
     }
     for(int i=0;i<=n;++i)
     if(i!=rt&&in[i]==INF)return -1;
     int cnt=-1;
     memset(id,-1,sizeof(id));
     memset(vis,-1,sizeof(vis));
     in[rt]=0;
     for(int i=0;i<=n;++i){
        ret+=in[i];
        int v=i;
        while(vis[v]!=i&&id[v]==-1&&v!=rt){
            vis[v]=i;
            v=pre[v];
        }
        if(v!=rt&&id[v]==-1){
            ++cnt;
            for(int u=pre[v];u!=v;u=pre[u])
              id[u]=cnt;
            id[v]=cnt;
        }
     }
     if(cnt==-1)break;
     for(int i=0;i<=n;++i)
     if(id[i]==-1)id[i]=++cnt;
     for(int i=1;i<=m;++i){
        int u=edge[i].u,v=edge[i].v;
        edge[i].u=id[u];
        edge[i].v=id[v];
        if(id[u]!=id[v])edge[i].w-=in[v];
     }
     n=cnt;
     rt=id[rt];
     }
     return ret;   
}
int main()
{
    while(~scanf("%d%d",&n,&m)){
        LL sum=0;
        for(int i=1;i<=m;++i){
          scanf("%d%d%I64d",&edge[i].u,&edge[i].v,&edge[i].w);
          ++edge[i].u,++edge[i].v;
          sum+=edge[i].w;
        }
        ++sum;
        for(int i=m+1;i<=m+n;++i){
           edge[i].u=0,edge[i].v=i-m;
           edge[i].w=sum; 
        }
        LL ans=zhuliu(0,n,n+m);
        if(ans-sum>=sum)printf("impossible\n\n");
        else printf("%I64d %d\n\n",ans-sum,pos-m-1);  
    }
    return 0;
}