answer.js

/**
 * Given an array of Person objects, returns the root PersonTreeNode (the CEO).
 * @param {Person[]} employees - An array of Person objects representing all the employees of the company.
 * @returns {PersonTreeNode} The CEO of the organization.
 */
function generateTree(employees) {
  let ceo = null

  const visitedById = new Map()
  const visitedByManagerId = new Map()

  for (const employee of employees) {
    const node = new PersonTreeNode(employee)
    visitedById.set(employee.id, node)

    // Add any direct reports we've already visited for this manager
    const visitedDirectReports = visitedByManagerId.get(employee.id)
    if (Array.isArray(visitedDirectReports)) {
      node.directReports.push(...visitedDirectReports)
    }

    // Check if this is the CEO and skip the remaining logic in the loop if so
    if (employee.manager === null) {
      ceo = node
      continue
    }

    const managerNode = visitedById.get(employee.manager.id)
    if (managerNode) {
      // Add our node to its manager's direct reports (if manager visited)
      managerNode.directReports.push(node)
    } else {
      // Otherwise add node to map to be added to the manager's direct reports once we visit the manager
      const visitedDirectReportsForManager = visitedByManagerId.get(
        employee.manager.id,
      )
      if (Array.isArray(visitedDirectReportsForManager)) {
        visitedDirectReportsForManager.push(node)
      } else {
        visitedByManagerId.set(employee.manager.id, [node])
      }
    }
  }

  return ceo
}