52分玄关求条，急!!

#include <bits/stdc++.h>
using namespace std;
int ans,n, k, siz[1200005], son[1200005], top[1200005], dep[1200005], fa[1200005], dis[1200005], a[1200005];
vector <int> G[1200005];
void dfs(int u, int f) {
	dep[u] = dep[f] + 1;
	siz[u] = 1;
	fa[u] = f;
	for (int i = 0; i < G[u].size(); i++) {
		int v = G[u][i];
		if (v == f) continue;
		dfs(v, u);
		siz[u] += siz[v];
		ans=max(dis[u]+dis[v]+1,ans);
		dis[u]=dis[v]+1;
	}
}
int main() {
	cin >> n;
	for (int i = 1; i <= n; i++) {
		cin >> a[i];
	}
	int u, v;
	for (int i = 1; i < n; i++) {
		cin >> u >> v;
		G[u].push_back(v), G[v].push_back(u);
	}
	dfs(1, 0);
	cout << ans;
	return 0;
}