http://codeforces.com/problemset/problem/275/D
minn[i]代表以i为根的子树要变零树要减几次 maxx[i]是加几次 最后汇总至根节点输出maxx[1]-minn[1]即可
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll N=0x3f3f3f3f3f3f3f3f;
const int maxn=1e5+10;
struct node
{
int v,next;
};
node edge[2*maxn];
ll val[maxn],minn[maxn],maxx[maxn];
int first[maxn];
int n,num;
void addedge(int u,int v)
{
edge[num].v=v;
edge[num].next=first[u];
first[u]=num++;
}
void dfs(int cur,int fa)
{
int i,v;
for(i=first[cur];i!=-1;i=edge[i].next){
v=edge[i].v;
if(v!=fa){
dfs(v,cur);
minn[cur]=min(minn[cur],minn[v]);
maxx[cur]=max(maxx[cur],maxx[v]);
}
}
val[cur]-=maxx[cur]+minn[cur];
if(val[cur]>0) maxx[cur]+=val[cur];
else minn[cur]+=val[cur];
}
int main()
{
int i,u,v;
scanf("%d",&n);
memset(first,-1,sizeof(first));
num=0;
for(i=1;i<=n-1;i++){
scanf("%d%d",&u,&v);
addedge(u,v);
addedge(v,u);
}
for(i=1;i<=n;i++){
scanf("%lld",&val[i]);
}
dfs(1,0);
printf("%lld\n",maxx[1]-minn[1]);
return 0;
}