toOutline.js

const Heading = function(weight, text) {
  this.weight = weight;
  this.text = text
};
const Node = function(heading, children) {
  this.heading = heading;
  this.children = children;
};

/**
 * Builds the whole node structure given a heading list.
 * 
 * @param {array} headings - The headings list.
 * @param {Node} parentNode - The current node.
 * @param {int} idx - The headings index.
 * @param {int} currentWeight - The current weight to compare
 * 
 * @return {Node} - A Node with nested children.
 */
function buildNodeStructure(headings, parentNode, idx, currentWeight) {
  for (let i = idx; i < headings.length; i++) {
    const headingWeight = headings[i].weight;

    if (headingWeight === currentWeight + 1) {
      const childNode = new Node(headings[i], []);
      parentNode.children.push(childNode);
      buildNodeStructure(headings, childNode, i + 1, childNode.heading.weight);
    }

    if (headingWeight === currentWeight || headingWeight === currentWeight - 1) {
      return;
    }
  }
  
  return parentNode;
}

/**
 * Converts a list of input headings into nested nodes
 * 
 * @param headings: Array of Headings as ordered in the input
 */
function toOutline(headings) {
  // Implement this function. Sample code below builds
  // an outline of only the first heading
  return buildNodeStructure(headings, new Node(new Heading(0, ''), []), 0, 0);
}

// Parses a line of the input
// This implementation is correct for all predefined test cases
function parse(record) {
  const firstSpace = record.indexOf(" ");
  const weight = parseInt(record.substr(1, firstSpace));
  const text = record.substr(firstSpace + 1);
  return new Heading(weight, text);
}

// Converts a node to HTML
function toHtml(node) {
  let childHtml = "";
  if (node.children.length > 0) {
    childHtml = `<ol>${node.children.map(child => `<li>${toHtml(child)}</li>`).join("\n")}</ol>`;
  }
  const heading = node.heading.text.length === 0 ? "" : node.heading.text + "\n";
  return heading + childHtml;
}

process.stdin.resume();
process.stdin.setEncoding("ascii");
var _input = "";
process.stdin.on("data", function (input) {
    _input += input;
});

process.stdin.on("end", function () {
  const headings = _input.split("\n").map(r => parse(r));
  const outline = toOutline(headings);
  console.log(toHtml(outline));
});