BZOJ 2521 最小生成树(最小割)

http://www.lydsy.com/JudgeOnline/problem.php?id=2521

题意:每次能增加一条边的权值1，求最小代价让一条边保证在最小生成树里

思路:如果两个点中有环，那么这条边必须不能是环的最大边，这样子把之前所有的边权值变成V+1-v[i]，无向图网络流就可以了

 #include<algorithm>
 #include<cstdio>
 #include<cmath>
 #include<cstring>
 #include<iostream>
 #define inf 0x7fffffff
 struct edge{
     int u,v,w;
 }e[];
 int n,m,id,dis[],cnt[],tot,S,T,nodes;
 int flow[],op[],next[],first[],go[];
 int read(){
     char ch=getchar();int t=,f=;
     while (ch<''||ch>''){if (ch=='-') f=-;ch=getchar();}
     while (''<=ch&&ch<=''){t=t*+ch-'';ch=getchar();}
     return t*f;
 }
 void insert(int x,int y,int z){
     tot++;
     go[tot]=y;
     next[tot]=first[x];
     first[x]=tot;
     flow[tot]=z;
 }
 void add(int x,int y,int z){
     insert(x,y,z);op[tot]=tot+;
     insert(y,x,);op[tot]=tot-;
 }
 void add2(int x,int y,int z){
     insert(x,y,z);op[tot]=tot+;
     insert(y,x,z);op[tot]=tot-;
 }
 int dfs(int x,int f){
     if (x==T) return f;
     int mn=nodes,sum=;
     for (int i=first[x];i;i=next[i]){
         int pur=go[i];
         if (flow[i]&&dis[pur]+==dis[x]){
             int save=dfs(pur,std::min(f-sum,flow[i]));
             sum+=save;
             flow[i]-=save;
             flow[op[i]]+=save;
             if (sum==f||dis[S]>=nodes) return sum;
         }
         if (flow[i]) mn=std::min(mn,dis[pur]);
     }
     if (sum==){
         cnt[dis[x]]--;
         if (cnt[dis[x]]==){
             dis[S]=nodes;
         }else{
             dis[x]=mn+;
             cnt[dis[x]]++;
         }
     }
     return sum;
 }
 int main(){
     n=read();m=read();id=read();
     S=,T=n+;nodes=n+;
     for (int i=;i<=m;i++){
         e[i].u=read();e[i].v=read();e[i].w=read();
     }
     add(S,e[id].u,inf);
     add(e[id].v,T,inf);
     for (int i=;i<=m;i++)if (i!=id&&e[i].w<=e[id].w){
         add2(e[i].u,e[i].v,e[id].w+-e[i].w);
     }
     int ans=;
     while (dis[S]<nodes) ans+=dfs(S,inf);
     printf("%d\n",ans);
 }