BZOJ 1827: [Usaco2010 Mar]gather 奶牛大集会 树形DP_带权重心

#include <bits/stdc++.h>
#define setIO(s) freopen(s".in","r",stdin) 
#define maxn 1000000 
#define inf 100000000000000 
#define ll long long 
using namespace std; 
ll f[maxn], siz[maxn], sumv[maxn]; 
int C[maxn], hd[maxn], to[maxn << 1], nex[maxn << 1], val[maxn << 1]; 
int n, edges, root; 
ll tot; 
void add(int u, int v, int c)
{
    nex[++edges] = hd[u], hd[u] = edges, to[edges] = v, val[edges] = c; 
}
void dfs(int u, int ff)
{
    siz[u] = C[u], f[u] = 0; 
    for(int i = hd[u]; i ; i = nex[i]) 
    {
        int v = to[i]; 
        if(v == ff) continue; 
        dfs(v, u);         
        siz[u] += siz[v];
        f[u] = max(f[u], siz[v]); 
    }               
    f[u] = max(f[u], tot - siz[u]); 
    if(f[u] < f[root]) root = u; 
}      
void calc(int u,int ff)
{
    siz[u] = C[u], sumv[u] = 0; 
    for(int i = hd[u]; i ; i = nex[i])
    {
        int v = to[i]; 
        if(v == ff) continue; 
        calc(v, u); 
        siz[u] += siz[v]; 
        sumv[u] += sumv[v] + val[i] * siz[v]; 
    }
}
int main()
{
    // setIO("input"); 
    scanf("%d",&n); 
    for(int i = 1; i <= n; ++i) scanf("%d",&C[i]), tot += C[i]; 
    for(int i = 1, u, v, c; i < n; ++i) 
    {
        scanf("%d%d%d",&u,&v,&c); 
        add(u, v, c); 
        add(v, u, c); 
    }
    f[0] = inf, root = 0, dfs(1, 0);
    memset(siz, 0, sizeof(siz)), calc(root, 0);  
    printf("%lld\n",sumv[root]); 
    return 0; 
}