蓝桥杯 结点选择

思路：

无根树转有根树 + 树形dp。

实现：

#include <iostream>
#include <cstdio>
#include <vector>
#include <cstring>
#include <algorithm>
using namespace std;

const int MAXN = 100000;
const int INF = 0x3f3f3f3f;

int n, x, y;
int dp[MAXN + 5][2], a[MAXN + 5], f[MAXN + 5], vis[MAXN + 5];
vector<int> G[MAXN + 5];

void trans(int root, int p)
{
    for (int i = 0; i < G[root].size(); i++)
    {
        int tmp = G[root][i];
        if (tmp != p)
            trans(tmp, f[tmp] = root);
    }
}

void build()
{
    for (int i = 1; i <= n; i++)
        G[i].clear();
    for (int i = 2; i <= n; i++)
    {
        G[f[i]].push_back(i);
    }
}

void dfs(int now)
{
    vis[now] = 1;
    for (int i = 0; i < G[now].size(); i++)
    {
        int tmp = G[now][i];
        if (!vis[tmp])
        {
            dfs(tmp);
        }
        dp[now][1] += dp[tmp][0];
        dp[now][0] += max(dp[tmp][1], dp[tmp][0]);
    }
    dp[now][1] += a[now];
}

int main()
{
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        scanf("%d", &a[i]);
    }
    for (int i = 0; i < n - 1; i++)
    {
        scanf("%d %d", &x, &y);
        G[x].push_back(y);
        G[y].push_back(x);
    }
    int ans = -INF;
    trans(1, -1);
    build();
    memset(dp, 0, sizeof(dp));
    memset(vis, 0, sizeof(vis));
    dfs(1);
    ans = max(ans, max(dp[1][0], dp[1][1]));
    printf("%d
", ans);
    return 0;
}