bzoj1827[Usaco2010 Mar]gather 奶牛大集会*

 1 #include <cstdio>
 2 #include <cstring>
 3 #include <algorithm>
 4 #include <queue>
 5 #define inc(i,j,k) for(int i=j;i<=k;i++)
 6 #define maxn 100010
 7 #define ll long long
 8 using namespace std;
 9 
10 inline int read(){
11     char ch=getchar(); int f=1,x=0;
12     while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1; ch=getchar();}
13     while(ch>=‘0‘&&ch<=‘9‘)x=x*10+ch-‘0‘,ch=getchar();
14     return f*x;
15 }
16 struct e{int t; ll w; int n;}es[maxn*2]; int g[maxn],ess;
17 void pe(int f,int t,ll w){es[++ess]=(e){t,w,g[f]}; g[f]=ess;}
18 ll ds[maxn],sz[maxn],fads[maxn],dep[maxn],c[maxn],sum; int n,ans,fa[maxn];
19 void dfs1(int x){
20     sz[x]=c[x]; ds[x]=0;
21     for(int i=g[x];i;i=es[i].n)if(es[i].t!=fa[x]){
22         fa[es[i].t]=x; dep[es[i].t]=dep[x]+es[i].w; dfs1(es[i].t);
23         sz[x]+=sz[es[i].t]; ds[x]+=ds[es[i].t]+sz[es[i].t]*es[i].w;
24     }
25 }
26 void dfs2(int x,ll a,ll b){
27     fads[x]=a+b*(sum-sz[x]); ll sm=fads[x];
28     for(int i=g[x];i;i=es[i].n)if(es[i].t!=fa[x])sm+=ds[es[i].t]+es[i].w*sz[es[i].t];
29     for(int i=g[x];i;i=es[i].n)if(es[i].t!=fa[x])dfs2(es[i].t,sm-(ds[es[i].t]+es[i].w*sz[es[i].t]),es[i].w);
30 }
31 int main(){
32     n=read(); inc(i,1,n)c[i]=read(),sum+=c[i];
33     inc(i,1,n-1){int x=read(),y=read(),z=read(); pe(x,y,z); pe(y,x,z);} dfs1(1); dfs2(1,0,0); 
34     ans=1; inc(i,2,n)if(fads[i]+ds[i]<fads[ans]+ds[ans])ans=i; printf("%lld",fads[ans]+ds[ans]); return 0;
35 }