[1,2,[3,4,[5]],6,[[7,8]]]
[1,2,3,4,5,6,7,8]

package zzz.nestedlist;

import static zzz.nestedlist.Node.node;

public class NestedList {

	public static void main(String[] args) {

		Node root = node(
			node(1),
			node(2),
			node(
				node(3),
				node(4),
				node(
					node(5))),
			node(6),
			node(
				node(
					node(7),
					node(8))));

		print(root);

		print(
			root.flat());
	}

	private static void print(Node root) {
		System.out.println(root.toString());
	}
}

package zzz.nestedlist;

import static java.util.Arrays.asList;

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

public class Node {

	boolean isLeaf;
	int val;
	List<Node> children;

	public static Node node(int val) {
		return leaf(val);
	}

	public static Node node(Node... nodes) {
		return branch(asList(nodes));
	}

	private static Node leaf(int val) {
		Node node = new Node();
		node.isLeaf = true;
		node.val = val;
		return node;
	}

	private static Node branch(List<Node> nodes) {
		Node node = new Node();
		node.isLeaf = false;
		node.children = nodes;
		return node;
	}

	public Node flat() {

		if (isLeaf)
			return node(leaf(val));

		List<Node> nodes = new ArrayList<>();
		for (Node n : children)
			nodes.addAll(n.flat().children);
		return branch(nodes);
	}

	@Override
	public String toString() {

		if (isLeaf)
			return val + "";

		String body = children.stream()
			.map(Object::toString)
			.reduce("", (a, b) -> a + "," + b);
		return "[" + body.substring(1) + "]";
	}
}