【树形DP/搜索】BZOJ 1827: [Usaco2010 Mar]gather 奶牛大集会

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<algorithm>
 4 #include<cmath>
 5  
 6 #define maxn 100001
 7  
 8 using namespace std;
 9  
10 long long tot=0,w[maxn],f[maxn],ans;
11  
12 inline int in()
13 {
14     int x=0;char ch=getchar();
15     while(ch<‘0‘||ch>‘9‘)ch=getchar();
16     while(ch<=‘9‘&&ch>=‘0‘)x=x*10+ch-‘0‘,ch=getchar();
17     return x;
18 }
19  
20 struct ed{
21     int to,c,last;
22 }edge[maxn*2];
23  
24 int last[maxn],cnt=0,a[maxn];
25  
26 bool vis[maxn];
27  
28 void add(int u,int v,int c)
29 {
30     edge[++cnt].to=v,edge[cnt].c=c,edge[cnt].last=last[u],last[u]=cnt;
31     edge[++cnt].to=u,edge[cnt].c=c,edge[cnt].last=last[v],last[v]=cnt;
32 }
33  
34 void DP(int pos)
35 {
36     w[pos]=a[pos];vis[pos]=1;
37     for(int i=last[pos];i;i=edge[i].last)
38     {
39         int u=edge[i].to;
40         if(vis[u])continue;
41         DP(u);
42         w[pos]+=w[u];
43         f[pos]+=w[u]*edge[i].c+f[u];
44     }
45 }
46  
47 void dfs(int poi,long long val)
48 {
49     vis[poi]=1;ans=min(ans,val);
50     for(int i=last[poi];i;i=edge[i].last)
51         if(!vis[edge[i].to])
52             dfs(edge[i].to,val+(tot-2*w[edge[i].to])*edge[i].c);
53 }
54  
55 int main()
56 {
57     int n,u,v,c;
58     n=in();
59     for(int i=1;i<=n;i++)a[i]=in(),tot+=a[i];
60     for(int i=1;i<n;i++)
61     {
62         u=in();v=in();c=in();
63         add(u,v,c);
64     }
65     DP(1);ans=f[1];
66     memset(vis,0,sizeof(vis));
67     dfs(1,f[1]);
68     printf("%lld",ans);
69     return 0;
70 }