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#include <bits/stdc++.h>
using namespace std;
const int M=10005;
const int N=10005;
#define ri register int
struct setdian{
    int to,val,net;
}bian[M];
int head[N],cent,dfn[N],low[N],leve,color[M],tim,stk[M],top,cost[M],ans;
void add(int a,int b,int c)
{
    bian[++cent].net=head[a],head[a]=cent;
    bian[cent].to=b;bian[cent].val=c;  //
}
void dfs1(int u,int f)
{
    low[u]=dfn[u]=++tim;
      stk[top++]=u;
    for(ri i=head[u];i;i=bian[i].net)
    {
        int v=bian[i].to;
        if(dfn[v]==0)
        {
            dfs1(v,i);
            low[u]=min(low[u],low[v]);
        }
        else if((i^1)!=f) low[u]=min(low[u],dfn[v]);
    }
    if(low[u]==dfn[u])
    {
          leve++;
        while(top>0)
        {
            int v=stk[--top];
            color[v]=leve;
            if(v==u) break;
        }
    }
}
int n,m;
int main(){
   while(scanf("%d%d",&n,&m)!=-1)  //qing 0 /zui da
   {
         memset(head,0,sizeof head);
         memset(stk,0,sizeof stk);
         memset(dfn,0,sizeof dfn);
         memset(low,0,sizeof low);
         memset(bian,0,sizeof bian);
         memset(color,0,sizeof color);
         top=cent=tim=leve=ans=0;
         for(ri i=0;i<=n;i++) cost[i]=INT_MAX;
        for(ri i=1;i<=m;i++)
        {
              int a,b,c;
              scanf("%d%d%d",&a,&b,&c);
              add(a,b,c);
     }
      for(ri i=0;i<n;i++)
      {
            if(dfn[i]==0) dfs1(i,0);
      }
      for(ri i=0;i<n;i++)
      for(ri j=head[i];j;j=bian[j].net)
      {   int v=bian[j].to,w=bian[j].val;
          if(color[v]!=color[i])
            {
                cost[color[v]]=min(cost[color[v]],w);
                //cout<<color[v]<<endl;
            }
      }
       //cout<<"lalal"<<" "<<color[0]<<endl;
      for(ri i=1;i<=leve;i++)
      {
           if(cost[i]!=INT_MAX&&i!=color[0])
              ans+=cost[i];
      }
      printf("%d\n",ans);
    }
    return 0;    
}

 

来源：https://www.cnblogs.com/Lamboofhome/p/11735350.html