2022 CCPC桂林 G. Group Homework

题意：

思路：

有时间再补，先插眼。

时间复杂度：$O（n）$

int n, m;
int h[N], e[N], ne[N], idx;
int s[N];
multiset<int> q[N];
multiset<int> d[N];
int ans;
vector<int> tmp;
int self[N];
int jinian[N];

void add(int a, int b) {
	e[idx] = b;
	ne[idx] = h[a], h[a] = idx++;
}

void dfs1(int u, int fa) {
	for (int i = h[u], cnt; ~i; i = ne[i]) {
		int j = e[i];
		if (j == fa)
			continue;

		dfs1(j, u);

		cnt = s[j];
		if (!q[j].empty())
			cnt += *(--q[j].end());

		q[u].insert(cnt);
		if (q[u].size() > 4)
			q[u].erase(q[u].begin());

	}
}

void dfs2(int u, int fa) {

	for (int i = h[u], cnt; ~i; i = ne[i]) {
		int j = e[i];
		if (j == fa)
			continue;

		dfs2(j, u);

		tmp.clear();
		for (auto k : q[j])
			tmp.push_back(k);
		reverse(all(tmp));
		cnt = s[j];
		for (int k = 0; k < 2 && k < (int)tmp.size(); k++)
			cnt += tmp[k];
		self[j] = cnt;

		if (!d[j].empty())
			self[j] = max(self[j], *(--d[j].end()));

		d[u].insert(self[j]);
		if (d[u].size() > 2)
			d[u].erase(d[u].begin());
	}
}

void dfs3(int u, int fa) {
	for (int i = h[u], cnt; ~i; i = ne[i]) {
		int j = e[i];
		if (j == fa)
			continue;

		cnt = s[j];
		if (!q[j].empty())
			cnt += *(--q[j].end());

		auto t = q[u].find(cnt);
		if (t != q[u].end())
			q[u].erase(t);

		int cha = s[u];
		if (!q[u].empty())
			cha += *(--q[u].end());

		q[j].insert(cha);
		if (q[j].size() > 4)
			q[j].erase(q[j].begin());

		q[u].insert(cnt);
		if (q[u].size() > 4)
			q[u].erase(q[u].begin());

		dfs3(j, u);
		jinian[j] = cnt;
	}
}

void dfs4(int u, int fa) {

	for (int i = h[u], cnt; ~i; i = ne[i]) {
		int j = e[i];
		if (j == fa)
			continue;

		dfs4(j, u);

		tmp.clear();
		for (auto k : q[u])
			tmp.push_back(k);
		reverse(all(tmp));
		cnt = 0;
		for (int k = 0; k < 3 && k < (int)tmp.size(); k++)
			cnt += tmp[k];
		ans = max(ans, cnt + s[u]);

		if (tmp.size() == 4)
			ans = max(ans, cnt + tmp[3]);
	}
}

void dfs5(int u, int fa, int pre) {

	for (int i = h[u], cnt; ~i; i = ne[i]) {
		int j = e[i];
		if (j == fa)
			continue;

		auto t = d[u].find(self[j]);
		if (t != d[u].end())
			d[u].erase(t);

		cnt = pre;
		if (!d[u].empty())
			cnt = max(cnt, *(--d[u].end()));

		t = q[u].find(jinian[j]);
		if (t != q[u].end())
			q[u].erase(t);

		tmp.clear();
		for (auto k : q[u])
			tmp.push_back(k);
		reverse(all(tmp));
		int wu = s[u];
		for (int k = 0; k < 2 && k < (int)tmp.size(); k++)
			wu += tmp[k];
		cnt = max(cnt, wu);

		ans = max(ans, cnt + self[j]);

		q[u].insert(jinian[j]);
		if (q[u].size() > 4)
			q[u].erase(q[u].begin());


		d[u].insert(self[j]);
		if (d[u].size() > 2)
			d[u].erase(d[u].begin());

		dfs5(j, u, cnt);
	}
}

signed main() {
	IOS
	sf(n);
	for (int i = 1; i <= n; i++) {
		h[i] = -1;
		sf(s[i]);
	}

	for (int i = 1, a, b; i < n; i++)
		sf2(a, b), add(a, b), add(b, a);

	if (n == 1) {
		pfn(0);
		return 0;
	}
	dfs1(1, 0);
	dfs2(1, 0);
	dfs3(1, 0);
	dfs4(1, 0);
	dfs5(1, 0, 0);

	pfn(ans);
	return 0;
}
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