树中距离之和

/*
 * 思路：
 * 1. 使用流构建树的邻接表表示。
 * 2. 使用两次深度优先搜索（DFS）来计算结果，配合流处理数据。
 */
import java.util.*;
import java.util.stream.*;

public class Solution {
    int[] answer, count;
    List<Set<Integer>> tree;
    int n;
    
    public int[] sumOfDistancesInTree(int N, int[][] edges) {
        n = N;
        tree = IntStream.range(0, n)
                         .mapToObj(i -> new HashSet<Integer>())
                         .collect(Collectors.toList());
        Arrays.stream(edges).forEach(e -> {
            tree.get(e[0]).add(e[1]);
            tree.get(e[1]).add(e[0]);
        });
        answer = new int[n];
        count = new int[n];
        Arrays.fill(count, 1);
        dfs(0, -1);
        dfs2(0, -1);
        return answer;
    }

    public void dfs(int node, int parent) {
        tree.get(node).stream()
            .filter(neighbor -> neighbor != parent)
            .forEach(neighbor -> {
                dfs(neighbor, node);
                count[node] += count[neighbor];
                answer[node] += answer[neighbor] + count[neighbor];
            });
    }

    public void dfs2(int node, int parent) {
        tree.get(node).stream()
            .filter(neighbor -> neighbor != parent)
            .forEach(neighbor -> {
                answer[neighbor] = answer[node] - count[neighbor] + n - count[neighbor];
                dfs2(neighbor, node);
            });
    }
}