Yes, you're right Paul. I still remember in verge (and excitement) of playing Pokemon GO ahead of time I have had installed it from a third party app store and man my phone had became a nightmare for me. Annoying ads will keep on popping, applications that I never heard of will be installed which I could even not remove even on factory reset. So had to change my phone itself. I am never going to do that again had my lesson learnt.
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I just use a recovery software access lost files on the Samsung phone, I think you can try the solution to recover your internal memory files simply.
Firstly connect the device with computer via usb cable and detect the phone quickly, then you can preview the deleted files and choose the ones to recover on your S2 phone.