This is nice article for approval via trigger http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_process_example.htm
I found we can use API to get record to approve from object 'ProcessInstanceWorkitem' and 'ProcessInstanceStep' See http://www.salesforce.com/us/developer/docs/api/Content/sforce_api_objects_processinstanceworkitem.htm
This is nice article for approval via trigger
http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_process_example.htm
I found we can use API to get record to approve from object 'ProcessInstanceWorkitem' and 'ProcessInstanceStep'
See http://www.salesforce.com/us/developer/docs/api/Content/sforce_api_objects_processinstanceworkitem.htm
Please check the link below for more info
https://success.salesforce.com/answers?id=90630000000hS9lAAE
Best Regards
Naga kiran