I've got it!; bingo!; that's it!
Exclamation of satisfaction when something goes as hoped or a plan succeeds. Casual and often used in self-congratulation.
I've got it! Now I can win.
Bingo! Just as I thought.
Derived from the verb 占める (しめる, 'to occupy' or 'to take possession'), with the past tense form しめた used idiomatically as an interjection expressing that one has secured a desired outcome.