Behair: I agree with previous comments, I could see H 18 of course but could'n't understand the interest of this problem
guile: The point is realizing how many liberties there are for each groups participating in the semeai. This understanding leads to choose the patient approach (which is the correct one) to only threaten the kill and eventually use the moves later.
JPortman: There must be a mistake in this problem. If the attacker plays at H18 as a first move ( "correct solution"), then the defender simply plays at D19 and makes a living group...
Ivan Detkov: The group is alive but H2 is the best move.
The point is realizing how many liberties there are for each groups participating in the semeai. This understanding leads to choose the patient approach (which is the correct one) to only threaten the kill and eventually use the moves later.