lightning:tree with account hierarchy recursion example

AccountData.cls

public class AccountData {
  @AuraEnabled public static Account[] getRecords() {
    return [SELECT Name, ParentId, (SELECT Name FROM Contacts) FROM Account];
  }
}

AccountHierarchy.app

<aura:application controller="AccountData" extends="force:slds">
  <aura:attribute name="data" type="List" default="[]" />
  <aura:handler name="init" value="{!this}" action="{!c.init}" />
  <lightning:tree header="Accounts" items="{!v.data}" />
</aura:application>

AccountHierarchyController.js

({
  init: function(component, event, helper) {
    var action = component.get("c.getRecords");
    action.setCallback(this, result => helper.parse(component, result));
    $A.enqueueAction(action);
  }
})

AccountHierarchyHelper.js

({
  parse: function(component, result) {
    var accounts = result.getReturnValue(),
        parents = { undefined: { items: [] }};
    accounts.forEach(account => parents[account.Id] = { items: [], name: account.Id, label: "Account: "+account.Name, expanded: false});
    accounts.forEach(account => { if(account.Contacts) { account.Contacts.forEach(contact => parents[account.Id].items.push({items: [], name: contact.Id, label: "Contact: "+contact.Name, expanded: false}))}});
    accounts.forEach(account => parents[account.ParentId].items.push(parents[account.Id]));
    component.set("v.data", parents[undefined].items);
  }
})