ReRoot Technique DP

void ReRoot (int v, int p) {
    res[v] = dp[v];
    for (auto u: adj[v]) {
        if (u == p) continue;
        dp[v] -= max(dp[u], 0);
        dp[u] += max(dp[v], 0);
        ReRoot (u, v);
        dp[u] -= max(dp[v], 0);
        dp[v] += max(dp[u], 0);
    }
    return;
}