Roll-up summary Using Trigger:-

Here 

ChildExample2__c  is child object and Amount__c is custom field in this object.

Example2__c  is parent object and Total_Amount__c is custom field in this object.

Roll-up summary means add all Amount fields in Child records , then update total amount with

the parent record.

Here we go

 

    trigger Testduplicate7 on ChildExample2__c (after delete, after insert, after update) {
    set<Id> Ids = new set<Id>();
    if(trigger.isInsert || trigger.isUpdate){
        for(ChildExample2__c p : trigger.new){
            Ids.add(p.Example2__c);
        }
    }
    if(trigger.isDelete){
        for(ChildExample2__c p : trigger.old){
            Ids.add(p.Example2__c);
        }
    }
    map<Id,Double> smap = new map <Id,Double>();
    for(AggregateResult q : [select Example2__c,sum(Amount__c)from ChildExample2__c where Example2__c IN :Ids group by Example2__c]){
        smap.put((Id)q.get('Example2__c'),(Double)q.get('expr0'));
    }
    List<Example2__c> ls = new List<Example2__c>();
    for(Example2__c a : [Select Id, Total_Amount__c from Example2__c where Id IN :Ids]){
        Double PipelineSum = smap.get(a.Id);
        a.Total_Amount__c = PipelineSum;
        ls.add(a);
    }
    update ls;
}

(OR) Another method here we go

/*Rollup summary on any field*/
trigger Test7 on ChildExample2__c (after insert,after update,after delete){
    if(Trigger.isinsert){
        Map<id,decimal> total = new Map<id,decimal>();
        for(ChildExample2__c ch :trigger.new){
            if(ch.amount__c!=null){
                if(total.get(ch.Example2__c)==null){
                    total.put(ch.Example2__c,ch.amount__c);
                }
                else
                {
                    decimal amt = 0;
                    amt = total.get(ch.Example2__c)+ch.amount__c;
                    total.put(ch.Example2__c,amt);
               
                }
            }
        }
        list<Example2__c> elist= new list<Example2__c>();
        list<Example2__c> exList = [select total_amount__c from Example2__c where id in:total.keyset()];
        for(Example2__c a:exList){
            if(a.total_amount__c==null){
                a.total_amount__c=0;
            }
            a.total_amount__c+=total.get(a.id);
            elist.add(a);
        }
        update elist;
    }
    if(Trigger.isUpdate){
        Map<id,decimal> total = new Map<id,decimal>();
        List<ChildExample2__c> chOld = Trigger.old;
        for(ChildExample2__c ch :trigger.New){
            if(ch.amount__c!=null){
                total.put(ch.Example2__c,ch.amount__c);
            }
            else{
                total.put(ch.Example2__c,0);
            }
        }
        Map<id,decimal> acc = new Map<id,decimal>();
        for(ChildExample2__c chod :trigger.old){
            if(chod.amount__c!=null){
                acc.put(chod.Example2__c,chod.amount__c);
            }
            else{
                acc.put(chod.Example2__c,0);
            }
        }
        list<Example2__c> elist= new list<Example2__c>();
        list<Example2__c> exList = [select total_amount__c from Example2__c where id in:total.keyset()];
        for(Example2__c a:exList){
            if(a.total_amount__c==null){
                a.total_amount__c=0;
            }
            System.Debug('==============sizw='+acc.get(a.id));
            a.total_amount__c+=total.get(a.id)-acc.get(a.id);
            elist.add(a);
        }
        update elist;
    }
    if(Trigger.isDelete){
        List<ChildExample2__c> chOld = Trigger.old;
        Map<id,decimal> acc = new Map<id,decimal>();
        for(ChildExample2__c chod :trigger.old){
            if(chod.amount__c!=null){
                acc.put(chod.Example2__c,chod.amount__c);
            }
        }
        list<Example2__c> elist= new list<Example2__c>();
        list<Example2__c> exList = [select total_amount__c from Example2__c where id in:acc.keyset()];
        for(Example2__c a:exList){
            if(a.total_amount__c==null){
                a.total_amount__c=0;
            }
            a.total_amount__c-=acc.get(a.id);
            elist.add(a);
        }
        update elist;
    }
}