public class OpportunityHelpers {
    public static void recalculateOpportunitiesAmount(List<Opportunity> oppList){

        Map<Id, Decimal> totalAmountForOppty = new Map<Id, Decimal>();

        for(OpportunityLineItem opptylineItem : [SELECT Id, OpportunityId, TotalPrice  FROM OpportunityLineItem WHERE Opportunityid IN :oppList]){
            if(!totalAmountForOppty.containsKey(opptylineItem.OpportunityId)){
                totalAmountForOppty.put(opptylineItem.OpportunityId, opptylineItem.TotalPrice);
            }
            else{
                 Decimal amount = totalAmountForOppty.get(opptylineItem.OpportunityId);
                 amount += opptylineItem.TotalPrice;
                 totalAmountForOppty.put(opptylineItem.OpportunityId, amount);
                }
        }

        for(Opportunity opp : oppList){
            opp.Amount = totalAmountForOppty.get(opp.Id);
        }

        update oppList;
    }
}