SDSC 2017 Day5 T2

//f[i]表示以i为根子树归零累加值，g{i]表示累减值
//肯定是先搞定底部再搞上面 
#include<bits/stdc++.h>
using namespace std;
const int maxn = 300010;
int n, a[maxn], st[maxn], tot = 0;
long long f[maxn], g[maxn];

struct node{
    int v, nxt;
} edge[2*maxn];

inline void in(int x, int y){
    edge[++tot].v = y;
    edge[tot].nxt = st[x];
    st[x] = tot;
}

inline void dfs(int x, int fa){
    for(int i = st[x]; i; i = edge[i].nxt){
        int to = edge[i].v;
        if(to == fa)    continue;
        dfs(to, x);
        f[x] = max(f[x], f[to]);
        g[x] = max(g[x], g[to]);
    }
    long long t = f[x] - g[x] + a[x];
    if(t > 0)   g[x] += t;
    else if(t < 0)  f[x] -= t;//注意t是负数，所以要减 
}

int main(){
    scanf("%d", &n);
    for(int i = 1; i <= n; i++)
        scanf("%d", &a[i]);
    for(int i = 1, x, y; i < n; i++){
        scanf("%d%d", &x, &y);
        in(x, y);
        in(y, x);
    }

    dfs(1, 0);

    printf("%lld\n", f[1]+g[1]);
    return 0;
}