扁平化多级双向链表

// Java Stream Solution
// 思路：
// 使用递归和流的思想，将每一个节点及其子节点展平成列表。

import java.util.ArrayList;
import java.util.List;

class Node {
    public int val;
    public Node prev;
    public Node next;
    public Node child;

    public Node(int val) {
        this.val = val;
        this.prev = null;
        this.next = null;
        this.child = null;
    }
}

public class Solution {
    public Node flatten(Node head) {
        List<Node> nodeList = new ArrayList<>();
        flattenToList(head, nodeList);
        Node pseudoHead = new Node(0);
        Node curr = pseudoHead;
        for (Node node : nodeList) {
            curr.next = node;
            node.prev = curr;
            curr = curr.next;
            curr.child = null;
        }
        pseudoHead.next.prev = null;
        return pseudoHead.next;
    }

    private void flattenToList(Node head, List<Node> nodeList) {
        if (head == null) return;
        nodeList.add(head);
        flattenToList(head.child, nodeList);
        flattenToList(head.next, nodeList);
    }
}