Trigger to copy child object picklist values to parent object separated by comma

I have picklist field “Opp_Location__c” on opportunity object and text field “Location_Type__c” on Account object. I want to copy the picklist values from all opportunities related to the account and display it on “Location_Type__c” field on account separated by comma.

Trigger UpdateAmount on Opportunity(after update){
Set<id> setAccId = new Set<id>();
Map<ID, Account> mapaccountvalue = new Map<ID, Account>();
for(Opportunity opp: Trigger.New)
{
if(opp.Opp_Location__c!=null && c.Opp_Location__c!=Trigger.OldMap.get(opp.id).Opp_Location__c
&& opp.accountid !=null ){
setAccId.add(c.accountid);
}
}

mapaccountvalue = new Map<id,Account> ([select id,Location_Type__c,(SELECT ID, Opp_Location__c FROM Opportunities) from Account where id IN: setAccId]);

for(Account acct:mapaccountvalue.values())
{
List<String> strType = new List<String>();
for(Opportunity oppty:acct.Opportunities)
{
strType.add(oppty.Opp_Location__c);
}
acct.Location_Type__c = String.join(strType,',');

}

update mapaccountvalue.values;
}

Point to Note:
How to join list of strings with a separator using Apex in Salesforce?
join(iterableObj, separator)
Joins the elements of the specified iterable object, such as a List, into a single String separated by the specified separator.

List <String> strList = new List < String > { 'Mango', 'Orange', 'Apple' };
String str = String.join( strList, ', ' );
system.debug( 'Joined with coma is ' + str );

Looping thorugh SOQL Subquery

for ( Account acc: [ SELECT Id, Location_Type__c,(SELECT id,Opp_Location__c FROM Opportunities ) FROM Account ] ) {
for ( Opportunity opp : acc.Opportunities ) {
}

Reference:

https://salesforce.stackexchange.com/questions/172030/trigger-to-copy-child-object-picklist-values-to-parent-object-separated-by-comma/327526#327526