Although at medium difficulty, this challenge isn't very complicated with some Java knowledge. The algorithm is simple enough, you can study the code yourself :P
```pythonalphabet = 'abcdefghijklmnopqrstuvwxyz_!@'flag = 'spbctf{'seed = 5for i in range(7, 23):    flag += alphabet[seed]    seed = seed * 3 % len(alphabet)flag += '}'print(flag)```
Flag: `spbctf{fpqt@_ucgszrwiyo}`