将 N 叉树编码为二叉树

/*
 * 思路：
 * 使用 Java Stream 编码 N 叉树为二叉树和解码二叉树为 N 叉树。
 * 
 * 主要步骤与非 Stream 版本相同，但我们将使用 Stream API 来处理孩子节点的编码和解码。
 */

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

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 TreeNode {
    public int val;
    public TreeNode left;
    public TreeNode right;

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

public class Codec {
    public TreeNode encode(Node root) {
        if (root == null) return null;

        TreeNode newRoot = new TreeNode(root.val);
        if (!root.children.isEmpty()) {
            newRoot.left = encode(root.children.get(0));
        }

        TreeNode current = newRoot.left;
        root.children.stream().skip(1).forEach(child -> {
            current.right = encode(child);
            current = current.right;
        });

        return newRoot;
    }

    public Node decode(TreeNode root) {
        if (root == null) return null;

        Node newRoot = new Node(root.val, new ArrayList<>());
        TreeNode current = root.left;

        Stream.iterate(current, Objects::nonNull, node -> node.right)
              .forEach(node -> newRoot.children.add(decode(node)));

        return newRoot;
    }
}