N 叉树的层序遍历

// 思路：
// 1. 使用广度优先搜索（BFS）遍历树的每一层。
// 2. 使用Java Stream API来简化部分操作。

import java.util.*;
import java.util.stream.Collectors;

class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    public Node(int val) {
        this.val = val;
    }

    public Node(int val, List<Node> children) {
        this.val = val;
        this.children = children;
    }
}

class Solution {
    public List<List<Integer>> levelOrder(Node root) {
        if (root == null) return Collections.emptyList();
        Queue<Node> queue = new LinkedList<>();
        queue.offer(root);
        List<List<Integer>> result = new ArrayList<>();

        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> level = new ArrayList<>();
            List<Node> nextLevel = new ArrayList<>();

            for (int i = 0; i < size; i++) {
                Node node = queue.poll();
                level.add(node.val);
                nextLevel.addAll(node.children);
            }

            result.add(level);
            queue.addAll(nextLevel);
        }

        return result;
    }
}