Smells like premature optimisation in here.

I know String concetenation is bad but never thought it'd add to time complexity. Saw this thread, it might come handy: https://www.reddit.com/r/learnprogramming/comments/4uh2b6/java_how_is_the_running_time_of_string/

this is bs. its O(n)

can u explain how concatenation make it O(n^2)

That's because it copies whole string on every iteration. It would be O(n) if you used StringBuilder.

Wait how is this O(n^2) its only 1 for loop? I had a similar solution using if/else instead of tenary but I am also running O(n) Time complexity

The purpose is to finish in one line, not to be readable. I think it's funny.

Bad solution! Time complexity is O(n^2), while there are more than one linear complexity solutions.