How to write a rollup summary trigger for a lookup relationship in Salesforce?

Hi Suraj

The link may can solves your problem:https://github.com/abhinavguptas/Salesforce-Lookup-Rollup-Summaries

You can also look in to this:https://andyinthecloud.com/2013/07/07/new-tool-declarative-rollups-for-lookups/

And if you have requirement for only one object ,you can prefer jeff solution as well:http://blog.jeffdouglas.com/2009/07/30/roll-up-summary-fields-with-lookup-relationships-part-1/

Hope it may help you