Trigger Scenario 6 In salesforce ROLLUP SUMMARY for COUNT:

//Trigger Best practice for count

trigger CountContacts on contact (after delete,after update, after insert, after undelete) {
 list<account> lstacc = new list<account>();
set<id> accids = new set<id>();
if(trigger.isInsert && trigger.isafter){
  for(contact con : Trigger.new){
      accids.add(con.accountid);
   }
}
else if(trigger.isDelete && trigger.isafter){
    for(contact con : Trigger.old){
        accids.add(con.accountid);
     }
}
else if(trigger.isUnDelete && trigger.isafter){
     for(contact con : Trigger.new){
     accids.add(con.accountid);
      }
}
else if(trigger.isUpdate && trigger.isafter){
     for(contact con : trigger.new){
if(trigger.oldmap.get(con.id).accountid!=con.accountid){
accids.add(con.accountid);
accids.add(trigger.oldmap.get(con.id).accountid);
}
}
}
AggregateResult[] groupedResults = [SELECT COUNT(Id), accountid FROM contact where accountid IN :accids GROUP BY accountid ];
   for(AggregateResult ar:groupedResults) {
        Id custid = (ID)ar.get('accountid');
        Integer count = (INTEGER)ar.get('expr0');
        account cust1 = new account(Id=custid);
        cust1.NumberofLocations__c = count;
        lstacc.add(cust1);
     }
     update lstacc;
}
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Trigger Scenario for Sum of Amount of all child to parent


trigger TestingContactTrigger on Contact (before update,after insert,after update,after delete,after undelete) {
    list<account> lstacc = new list<account>();
    set<id> ids = new set<id>();
    if(Trigger.isafter && (trigger.isinsert || trigger.isupdate || trigger.isundelete)){
        for(contact con : trigger.new){
            ids.add(con.accountid);
        }
    }
    if(trigger.isafter && trigger.isdelete){
        for(contact con : trigger.old){
            ids.add(con.AccountId);
        }
    }
    list<aggregateresult> lstagr =[Select SUM(Amount__c) totalSum, accountid From contact Where accountid IN : Ids GROUP BY accountid];
    for(aggregateresult ag : lstagr){
        id accid = (ID)ag.get('accountid');
        double amount = (DOUBLE)ag.get('totalsum');
        account acc = new account(id = accid);
        acc.Total__c = amount;
        lstacc.add(acc);
    }
    update lstacc;
}

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---------------------------------------------------------------------Another way-----
Trigger triggername on contact(after insert,after update,after delete,after undelete){
Set<id> accids = new set<id>();
    map<id,list<contact>> mapcon = new map<id,list<contact>>();
    if(trigger.isafter &&(trigger.isinsert ||trigger.isupdate || trigger.isundelete)){
        for(contact con : trigger.new){
            accids.add(con.AccountId);
        }
    }
    if(trigger.isafter && trigger.isdelete){
        for(contact con : trigger.old){
            accids.add(con.AccountId);
        }
    }
 
 
  //  list<contact> lstcon = [select id,lastname,amount1__c,accountid from contact where accountid IN: accids];
    //   integer count = lstcon.size();
     
    list<account> lstacc = new list<account>();
    for(account acc :[select id,name,NumberofLocations__c,(select id from contacts)from account where id IN:accids]){
       acc.NumberofLocations__c = acc.contacts.size();
        lstacc.add(acc);
    }
    update lstacc;
}
----------------------------------------------------
Rollup summary Amount

trigger RollupOppAmountTrigger on Opportunity (after insert,after update,after delete,after undelete) {
       Map<Id, List<Opportunity>> acctIdOpptyListMap = new Map<Id, List<Opportunity>>();
       Set<string> acctIds = new Set<string>();
    if(trigger.isUpdate || trigger.isInsert || trigger.isundelete){
        for(Opportunity oppty : trigger.New){
                acctIds.add(oppty.AccountId);
        }  
    }
    if(trigger.isDelete){
        for(Opportunity oppty : trigger.old){
              acctIds.add(oppty.AccountId);
        }  
    }
    if(acctIds.size() > 0){
        for(Opportunity oppty : [SELECT Amount, AccountId FROM Opportunity WHERE AccountId IN : acctIds]){
            if(!acctIdOpptyListMap.containsKey(oppty.AccountId)){
                acctIdOpptyListMap.put(oppty.AccountId, new List<Opportunity>());
            }
            acctIdOpptyListMap.get(oppty.AccountId).add(oppty);
        }
        List<Account> acctList = new List<Account>();
        for(Account acct : [SELECT NumberofLocations__c  FROM Account WHERE Id IN: acctIds]){
            List<Opportunity> tempOpptyList = new List<Opportunity>();
            tempOpptyList = acctIdOpptyListMap.get(acct.Id);
            Double totalOpptyAmount = 0;
            for(Opportunity oppty : tempOpptyList){
                if(oppty.Amount != null){
                    totalOpptyAmount += oppty.Amount;
                }
            }
            acct.NumberofLocations__c  = totalOpptyAmount;
           acctList.add(acct);
        }
        update acctList;
    }
}