hdu 2121

本题为无固定根的最小树形图

#include<cstring>#include<cstdio>#include<cmath>#include<algorithm>#define N 1010#define M 20010#define inf 100000000using namespace std;int a[N],b[N],c[N];int in[N],pre[N],id[N],vis[N];struct Edge{    int u,v,w;}edge[M];int m,n,ansi;int Directed_MST(int root){    int ret=0,i;    while(1){        for(i=0;i<=n;i++)in[i]=inf;        for(i=0;i<m;i++){            int u=edge[i].u,v=edge[i].v;            if(edge[i].w<in[v] && u!=v){                pre[v]=u;                in[v]=edge[i].w;                if(u==root)                    ansi=i;  //这里确定了虚拟根节点的出边，假设有k个点都可以做真正根节点，那么最后这k个点一定在一个环里            }        }        in[root]=0;        for(i=0;i<=n;i++)            if(in[i]==inf)return -1;        int node=0;        memset(id,-1,sizeof(id));        memset(vis,-1,sizeof(vis));        for(i=0;i<=n;i++){            ret+=in[i];            int v=i;            while(vis[v]!=i && id[v]==-1 && v!=root){                vis[v]=i;                v=pre[v];            }            if(v!=root && id[v]==-1){                for(int u=pre[v];u!=v;u=pre[u])                    id[u]=node;                id[v]=node++;            }        }        if(node==0)break;        for(i=0;i<=n;i++)            if(id[i]==-1)                id[i]=node++;        for(i=0;i<m;i++){            int v=edge[i].v;            edge[i].u=id[edge[i].u];            edge[i].v=id[edge[i].v];            if(edge[i].u!=edge[i].v)                edge[i].w-=in[v];        }        n=node-1;        root=id[root];    }    return ret;}int main(){    int i,j,sum,vv,e;    int u,v,w;    while(scanf("%d %d",&vv,&e)==2){        sum=1;        for(i=0;i<e;i++){            scanf("%d %d %d",&u,&v,&w);            u++,v++;            edge[i].u=u,edge[i].v=v,edge[i].w=w;            sum+=w;        }        for(i=0;i<vv;i++)            edge[i+e].u=0,edge[i+e].v=i+1,edge[i+e].w=sum;        m=vv+e,n=vv;        int ans=Directed_MST(0);        if(ans>sum*2)printf("impossible\n\n");        else printf("%d %d\n\n",ans-sum,ansi-e);    }}