tl;dr
Patch `checkFlag()` return values and brute force flag char by char.

full writeup - https://github.com/elklepo/pwn/blob/master/CONFidenceCTF_Teaser_2019/Elementary/writeup.md

Original writeup (https://github.com/elklepo/pwn/blob/master/CONFidenceCTF_Teaser_2019/Elementary/writeup.md).