The afterword says there was some editorial pressure to keep the ending heavy, while the author wanted to make it happy. Thus the ambiguous ending and the "OR NOT" between the two options.
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@Nicromatic , I think this kind of thing is cheap, a way of the author to please everyone by giving them both endings. I saw that in a recent Korean movie, the director shoehorned both possible endings, too. This just shows me they don't have the guts to choose one ending that reflects how they feel about their own story, and bow to outside pressure instead of following through with it.
Since he did say in the afterword that he wanted the happy ending, I'll consider the ice cream run the official ending. It's more surprising, too, since I was so sure this would end in tragedy. After sorting out the matter with the father, and the three friends reuniting, and all the mental strength Jaekyung showed by refusing to let himself be dominated by the ants, reversing course in the last few pages and forcing a tragic ending doesn't cut it, anyway.