Roll Up Contact Count on Account

trigger rollupCount on Child__c (after insert, after delete) 
{ 
List masterList = new List();
Map masterMap = new Map();
for(List mstrList : [Select id, Trigger_Rollup_Count__c from Master__c])
{ 
for(Master__c master : mstrList)

{
masterMap.put(master.Id, master); 
}
}
if(trigger.isInsert)
{
for(Child__c child : trigger.new)
{ 
 
if(masterMap.containskey(child.Master__c)){
if(masterMap.get(child.Master__c).Trigger_Rollup_Count__c != null){

Master__c mas = masterMap.get(child.Master__c);

mas.Trigger_Rollup_Count__c = mas.Trigger_Rollup_Count__c + 1; 
 
masterList.add(mas); 
 
}
else{
Master__c mas = masterMap.get(child.Master__c);
mas.Trigger_Rollup_Count__c = 1; masterList.add(mas); 
} 
} 
} 
} 
if(trigger.isDelete)
{
for(Child__c child : trigger.Old)
{ 
if(masterMap.containskey(child.Master__c))
{ 
Master__c mas = masterMap.get(child.Master__c); 
mas.Trigger_Rollup_Count__c = mas.Trigger_Rollup_Count__c - 1;
masterList.add(mas);
}
} 
} 
if(masterList.size() > 0)
{
update masterList; 
} 
}