POJ 2377

水一发，最小生成树。注意不连通情况。

#include <cstdio> 
#include <iostream> 
#include <cstring>  
#include <cctype>  
#include <algorithm>  
#define LL unsigned __int64
using namespace std; 

const int N= 1010;
const int M= 20100;
struct Edge{
    int u,v,c;
    int next;
}edge[M*2];
int head[N];
int n,m,tot;
bool vis[N];
int val[N];

void addedge(int u,int v,int c){
    edge[tot].u=u;
    edge[tot].v=v;
    edge[tot].c=c;
    edge[tot].next=head[u];
    head[u]=tot++;
}

void MST(){
    int u=1,v,mm,k;
    val[u]=0;
    for(int i=1;i<n;i++){
        vis[u]=true;
        for(int e=head[u];e!=-1;e=edge[e].next){
            v=edge[e].v;
            if(!vis[v]){
                val[v]=min(val[v],edge[e].c);
            }
        }
        mm=1,k=-1;
        for(int j=1;j<=n;j++){
            if(!vis[j]&&val[j]<mm){
                mm=val[j];
                k=j;
            }
        }
        if(k==-1) return ;
        u=k;
    }
}

int main(){
    int u,v,c;
    while(scanf("%d%d",&n,&m)!=EOF){
        tot=0;
        for(int i=1;i<=n;i++){
            head[i]=-1;
            vis[i]=false,val[i]=1;
        }
        for(int i=0;i<m;i++){
            scanf("%d%d%d",&u,&v,&c);
            addedge(u,v,-c);
            addedge(v,u,-c);
        }
        MST();
        int ans=0,i;
        
        for(i=1;i<=n;i++){
            if(val[i]==1) break;
            ans+=val[i];
        }
        printf("%d\n",i>n?-ans:-1);
    }
    return 0;
}