2016夏季练习——最小树形图

来源：UVA11183

朱刘算法求最小树形图

代码：

#include <iostream>
#include <cstdio>
#include <string>
#include <cstring>
#include <algorithm>
#include <cmath>
using namespace std;
const int INF = 0x3f3f3f3f;
const int MAXE = 40000+10;
const int MAXV = 1010;
struct Edge{
    int u,v;
    int cost;
}edge[MAXE];
int n,m;
int mp[MAXV][MAXV];
int pre[MAXV],id[MAXV],vis[MAXV],in[MAXV];
int zhuliu(int root,int n,int m){
    int ans = 0;
    int u,v;
    while(1){//朱刘算法要求一直做到没有有向环
        for(int i=0;i<n;i++){
            in[i] = INF;
        }
        for(int i=0;i<m;i++){
            if(edge[i].u!=edge[i].v&&edge[i].cost<in[edge[i].v])
            {
 //               cout<<"in"<<endl;
                pre[edge[i].v] = edge[i].u;
                in[edge[i].v] = edge[i].cost;
            }
        }
        for(int i=0;i<n;i++)
            if(i!=root&&in[i]==INF)
                return -1;//判联通
        int tn=0;
        memset(id,-1,sizeof(id));
        memset(vis,-1,sizeof(vis));
        in[root] = 0;
        for(int i=0;i<n;i++){
            ans+=in[i];
            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] = tn;
                }
                id[v]=tn++;
            }
        }
        if(tn==0) break;
        for(int i=0;i<n;i++){
            if(id[i]==-1){
                id[i]=tn++;
            }
        }
        for(int i=0;i<m;){
            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++].cost-=in[v];
            }
            else{
                swap(edge[i],edge[--m]);
            }
        }
        n=tn;
        root = id[root];
    }
    return ans;
}
int main(){
    int T,nc=1;
    scanf("%d",&T);
    while(T--){
        scanf("%d%d",&n,&m);
        //cout<<n<<" "<<m<<endl;
        for(int i=0;i<n;i++){
            for(int j=0;j<n;j++){
                mp[i][j]=INF;
            }
        }
        int u,v,w;
        for(int i=0;i<m;i++){
            scanf("%d%d%d",&u,&v,&w);
            if(u==v) continue;
            mp[u][v] = min(mp[u][v],w);
        }
        int cnt = 0;
        for(int i=0;i<n;i++){
            for(int j=0;j<n;j++){
                if(mp[i][j]<INF){
                 //   cout<<"xianin ";
                    edge[cnt].u=i;
                    edge[cnt].v=j;
                    edge[cnt++].cost=mp[i][j];
                 //   cout<<i<<" "<<j<<" "<<mp[i][j]<<endl;
                }
            }
        }
        int ans = zhuliu(0,n,cnt);
        cout<<"Case #"<<nc++<<": ";
        if(ans==-1) cout<<"Possums!"<<endl;
        else cout<<ans<<endl;
    }
	return 0;
}