Last line should be return null, per the problem description.

4-liner, cheating on whitespace doesn't count.

Nice solution!

thanks, Markus! nice one, as well :) I think I just need more practice with the streams to come to that level

And another one here :-)

yes

EDIT: "one liner" in O(N) here (for fun)

nice one-liner! very elegant! I think it needs O(n^2) time though?

Please avoid to use cryptic and short variable names as a good practice.

вместо String symbol = s.substring(i, i+1);
лучше s.charAt(i)