How do i play the game in a new smartphone?

i buy a new smartphone and reset my old smartphone. i forget my google play account name. need i buy the game again? or can you find my google play account name in your server?

Only yourself and Google knows your GooglePlay account name. But it is always an email address that you have. So you can try all email addresses you use and chances are that one of them is the one.
Search your inboxes for emails from Google. The account with receipts is your best bet.
Good luck!