Added test for a more extreme case of manhattan distances.
Added isGoal test and implementation.
Refactored the validations to 0 so it become a constant and improve readibility.
Implemented the ToString method.
Implemented the twin method.
This method swaps the first non zero values in the puzzle.
Test and implementation of the equals method.
test and First implementation of the neighbors method.
It works for the top left corner, it has to be generalized to compute every posible move no matter where the 0 currently is.
Test and implementation of neighbors method.