Document management system - build unordered list (table of contents) recursively, Java

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//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;

class Rextester
{  
    public static void main(String args[])
    {
        
        List<Heading> headings = new ArrayList<>();
        headings.add(new Heading(1,"America"));
        headings.add(new Heading(2,"North America"));
        headings.add(new Heading(3,"United States"));
        headings.add(new Heading(2,"South America"));
        headings.add(new Heading(3,"Brazil"));
        headings.add(new Heading(3,"Urugway"));
        Node outline = toOutline(headings);
        String html = toHtml(outline);
        System.out.println(html);
    }
    
    /////////// BEGIN EDITABLE //////////////
    private static Node toOutline(List<Heading> headings) {
        // Implement this function. Sample code below builds an
        // outline of only the first heading.

Node root = new Node(new Heading(0, ""));
        
        addChildren(root,headings,0);
        
        return root;
    }
    
    private static void addChildren(Node node,List<Heading> headings,int i){
        int curr_weight = node.heading.weight;
        while(i<headings.size()){
            if(headings.get(i).weight==curr_weight || headings.get(i).weight==curr_weight-1)
                return;
            if(headings.get(i).weight==curr_weight+1){
                Node child = new Node(headings.get(i));
                node.children.add(child);
                addChildren(child,headings,i+1);
            }
            i++;
        }
    }
    /////////// END EDITABLE //////////////
    
    
    private static Heading parse(String record) {
        String[] parts = record.split(" ", 2);
        int weight = Integer.parseInt(parts[0].substring(1));
        Heading heading = new Heading(weight, parts[1].trim());
        return heading;
    }
    
    private static String toHtml(Node node) {
        StringBuilder buf = new StringBuilder();
        if (!node.heading.text.isEmpty()) {
            buf.append(node.heading.text);
            buf.append("\n");
        }
        Iterator<Node> iter = node.children.iterator();
        if (iter.hasNext()) {
            buf.append("<ul>");

while (iter.hasNext()) {
                Node child = iter.next();
                buf.append("<li>");
                buf.append(toHtml(child));
                buf.append("</li>");
                if (iter.hasNext()) {
                    buf.append("\n");
                }
            }
            buf.append("</ul>");
        }
        return buf.toString();
    }
    
}
 class Heading {
        protected int weight;
        protected String text;

public Heading(int weight, String text) {
            this.weight = weight;
            this.text = text;
        }
    }
     class Node {
        protected Heading heading;
        protected List<Node> children;

public Node(Heading heading) {
            this.heading = heading;
            this.children = new ArrayList<>();
        }
    }

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Compilation time: 0.73 sec, absolute running time: 0.15 sec, cpu time: 0.13 sec, memory peak: 18 Mb, absolute service time: 0,88 sec

Document management system - build unordered list (table of contents) ...