CCA的子树（dfs）

#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int N=2e5+100;
const ll inf=1e18;
ll w[N];
ll mx[N];
ll sum[N];
vector<int> v[N];
ll ans=-inf;
void dfs(int u,int fa){
	ll maxn=-inf;
	sum[u]=w[u];
	for(int i=0;i<v[u].size();i++){
		int j=v[u][i];
		if(j==fa) continue;
		dfs(j,u);
		sum[u]+=sum[j];
		if(maxn!=-inf) ans=max(ans,maxn+mx[j]);
		maxn=max(mx[j],maxn);
	}
	mx[u]=max(maxn,sum[u]);
}
int main(){
	int n;scanf("%d",&n);
	for(int i=1;i<=n;i++){
		scanf("%lld",&w[i]);
	}
	for(int i=1;i<n;i++){
		int a,b;scanf("%d%d",&a,&b);
		v[a].push_back(b);
		v[b].push_back(a);
	}
	dfs(1,0);
	if(ans==-inf) printf("Error\n");
	else printf("%lld\n",ans);
	return 0;
}