BZOJ 1827 洛谷 2986 [USACO10MAR]伟大的奶牛聚集Great Cow Gather

 1 #include<cstdio>
 2 #include<algorithm>
 3 #define N 200010
 4 #define rg register
 5 #define LL long long
 6 using namespace std;
 7 int n,tot,last[N],v[N];
 8 LL ans,sum,size[N],dis[N];
 9 struct edge{
10     int to,pre,dis;
11 }e[N];
12 inline int read(){
13     int k=0,f=1; char c=getchar();
14     while(c<‘0‘||c>‘9‘)c==‘-‘&&(f=-1),c=getchar();
15     while(‘0‘<=c&&c<=‘9‘)k=k*10+c-‘0‘,c=getchar();
16     return k*f;
17 }
18 void dfs(int x,int fa){
19     size[x]=v[x];
20     for(rg int i=last[x],to;i;i=e[i].pre)if((to=e[i].to)!=fa){
21         dis[to]=dis[x]+e[i].dis; 
22         dfs(to,x); size[x]+=size[to];
23     }
24 }
25 void solve(int x,int fa,LL now){
26     ans=min(ans,now);
27     for(rg int i=last[x],to;i;i=e[i].pre)if((to=e[i].to)!=fa){
28         LL tmp=now-size[to]*e[i].dis+(sum-size[to])*e[i].dis;
29         solve(to,x,tmp);
30     }
31 }
32 int main(){
33     n=read(); 
34     for(rg int i=1;i<=n;i++) v[i]=read(),sum+=v[i];
35     for(rg int i=1;i<n;i++){
36         int u=read(),v=read(),w=read();
37         e[++tot]=(edge){v,last[u],w}; last[u]=tot;
38         e[++tot]=(edge){u,last[v],w}; last[v]=tot;
39     }
40     dfs(1,0);
41     for(rg int i=1;i<=n;i++) ans+=dis[i]*v[i];
42     solve(1,0,ans);
43     printf("%lld\n",ans);
44     return 0;
45 }