account1
    >contact1
          >Contact 2
   account 2
        >contact 3
             >contact 4

<aura:component controller="TreeController" implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,forceCommunity:availableForAllPageTypes,force:lightningQuickAction" access="global" >
    <aura:handler name="init" value="{!this}" action="{!c.doInit}" />
    <aura:attribute name="data" type="List" default="[]"/>
    <lightning:tree header="Accounts" class="slds-is-selected" items="{!v.data}" onselect="{!c.handleSelect}"/>
</aura:component>

({
doInit : function(component, event, helper) {
console.log("Going here");
var action = component.get("c.getAccountList");
 action.setParams({ accountId : component.get("v.recordId")});
 //action.setCallback(this, result => helper.parse(component, result));
 action.setCallback(this, function(response) {
   var state = response.getState();
   console.log('state'+state);
   console.log('response'+JSON.stringify(response.getReturnValue()));
   if (state === "SUCCESS") {
     var result = response.getReturnValue();
     helper.parse(component, result);
     console.log('result----'+JSON.stringify(result));
   }
 });
 $A.enqueueAction(action);
 },
    handleSelect: function (component, event, helper) {
    //return name of selected tree item
    var myName = event.getParam('name');
    alert("You selected: " + myName);
    }
})

({
  parse: function(component, result) {
    var accounts = result;
    console.log('accounts'+JSON.stringify(accounts));
    var parents = { undefined: { items: [] }};
    var parentcontact = { undefined: { items: [] }};
    console.log('parentsA'+JSON.stringify(parents));
    accounts.forEach(account => parents[account.Id] = { items: [], name: account.Id, label:account.Name, expanded: false});
    console.log('parentsB'+JSON.stringify(parents));
    accounts.forEach(account => { if(account.Contacts) { account.Contacts.forEach(contact => parents[account.Id].items.push(parentcontact[contact.Id] = { items: [], name: contact.Id, label:contact.Name, expanded: false}))}});
    console.log('parentsC'+JSON.stringify(parents));
    accounts.forEach(account => { if(account.Contacts) { account.Contacts.forEach(contact => parents[account.Id].items.push( parentcontact[contact.parentId__c].items.push(parentcontact[contact.Id])))}});
    //account.Contacts.forEach(contact =>parentcontact[contact.parentId__c].items.push(parentcontact[contact.Id] ));
    console.log('parentsD'+JSON.stringify(parents));
    accounts.forEach(account => parents[account.ImmediateParent__c].items.push(parents[account.Id]));
    console.log('parentscontact'+JSON.stringify(parentcontact));
    console.log('parentsE'+JSON.stringify(parents[undefined].items));
    component.set("v.data",parents[undefined].items);
  }
})

@AuraEnabled
public static List<Account> getAccountList(Id accountId) {
 List<Account> finalvalue = new List<Account>();
 for(Account accA : [ Select Id, Name,(SELECT Id,Contact.Name,parentId__c, Phone FROM contacts),ParentId,ImmediateChildFlag__c,ImmediateParent__c,ImmediateParent__r.Name From Account Where ParentId =: accountId or Name LIKE'TreeAccount%']) {
      finalvalue.add(accA);
     }
 System.debug('finalvalue:::' + finalvalue);
 return finalvalue;
}