BZOJ 1927 星际竞速

http://www.lydsy.com/JudgeOnline/problem.php?id=1927

思路:把一个点拆成两个点，

S->i 费用0，流量1 (代表这个点可以移动到其他点所必备的流量)

i+n->T 费用0，流量1 (每个点都必须要走过)

u->v+n 费用w,流量1  (代表可以移动到那个点)

S->i+n 费用a[i],流量1 (代表从这个点瞬移)

 #include<algorithm>
 #include<cstdio>
 #include<cmath>
 #include<cstring>
 #include<iostream>
 int tot,go[],next[],first[],cost[],flow[];
 int op[],dis[],c[],vis[],edge[],from[];
 int S,T,n,m,ans;
 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,int l){
     tot++;
     go[tot]=y;
     next[tot]=first[x];
     first[x]=tot;
     flow[tot]=z;
     cost[tot]=l;
 }
 void add(int x,int y,int z,int l){
     insert(x,y,z,l);op[tot]=tot+;
     insert(y,x,,-l);op[tot]=tot-;
 }
 bool spfa(){
     for (int i=S;i<=T;i++)
      dis[i]=0x3f3f3f3f,vis[i]=;
     int h=,t=;c[]=S;vis[S]=;dis[S]=;
     while (h<=t){
         int now=c[h++];
         for (int i=first[now];i;i=next[i]){
             int pur=go[i];
             if (dis[pur]>dis[now]+cost[i]&&flow[i]){
                 dis[pur]=dis[now]+cost[i];
                 edge[pur]=i;
                 from[pur]=now;
                 if (vis[pur]) continue;
                 vis[pur]=;
                 c[++t]=pur;
             }
         }
         vis[now]=;
     }
     return dis[T]!=0x3f3f3f3f;
 }
 void updata(){
     int mn=0x7ffffff;
     for (int i=T;i!=S;i=from[i]){
         mn=std::min(mn,flow[edge[i]]);
     }
     for (int i=T;i!=S;i=from[i]){
         ans+=mn*cost[edge[i]];
         flow[edge[i]]-=mn;
         flow[op[edge[i]]]+=mn;
     }
 }
 int main(){
     n=read();m=read();
     S=;T=n+n+;
     for (int i=;i<=n;i++)
      add(S,i,,);
     for (int i=;i<=n;i++){
         int x=read();
         add(S,i+n,,x);
     }
     for (int i=;i<=n;i++)
         add(i+n,T,,);
     while (m--){
         int u=read(),v=read(),w=read();
         if (u>v) std::swap(u,v);
         add(u,v+n,,w);
     }
     ans=;
     while (spfa()) updata();
     printf("%d\n",ans);
 }