We are presented a string in brainfuck language. The first thing we notice that it is not outputting anything (not a single '.' character in it).
>+++++++++++[<++++++++++>-]<[-]>++++++++[<++++++>-]<[-]>++++++++[<++++++>-]<[-]
>++++++++++++++[<+++++++>-]<[-]>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++[<++>-]<[-]
>+++++++++++++++++++++++++++++++++++++++++[<+++>-]<[-]>+++++++[<+++++++>-]<[-]
>+++++++++++++++++++[<+++++>-]<[-]>+++++++++++[<+++++++++>-]<[-]>+++++++++++++[<++++>-]<[-]
>+++++++++++[<++++++++++>-]<[-]>+++++++++++++++++++[<+++++>-]<[-]>+++++++++++[<+++++++++>-]<[-]
>++++++++[<++++++>-]<[-]>++++++++++[<++++++++++>-]<[-]>+++++++++++++++++[<+++>-]<[-]
>+++++++++++++++++++[<+++++>-]<[-]>+++++++[<+++++++>-]<[-]>+++++++++++[<++++++++++>-]<[-]
>+++++++++++++++++++[<+++++>-]<[-]>++++++++++++++[<+++++++>-]<[-]
>+++++++++++++++++++[<++++++>-]<[-]>+++++++++++++[<++++>-]<[-]>+++++++[<+++++++>-]<[-]
>+++++++++++[<++++++++++>-]<[-]>+++++++++++++++++[<++++++>-]<[-]>+++++++[<++++++>-]<[-]
>+++++++++++[<+++++++++>-]<[-]
>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++[<+>-]<[-]
>+++++++++++[<+++>-]<[-]>+++++++++++++++++++++++++[<+++++>-]<[-]
The next thing that jumps into ones eye are these <[-]. The < jumps right back into the previous memory cell while [-] is equal to
while (cell_value[i] != 0) {
cell_value[i]--;
}
and causes whatever value was written into that cell before to be decreased to 0 again.
Since we are curious we want to output each of these values before they get neutralized again. So our task here is to simply insert a . command into each of the <[-] commands: <.[-]
Putting the resulting brainfuck code into a brainfuck interpreter will now give us the flag.