What does it mean to be surrounded? Whatever I think it means, it is contradicted by the fact that the '0' on the bottom row is not changed.
haha, i haven't solved it yet but it is trying to say that convert 0 to x only if it is bounded by Xs (bounded means that it has X on all 4 sides of it) in a 4-connected manner. I guess you are thinking of 8-connectivity and hence got confused.
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