...
Once your solve() method is working, clicking the Solve button will attempt to solve the board from its current configuration. The result will be a pop-up saying that the board is either solved or unsolvable.
During the middle of your implementation, after you have implemented the reduceBoard but before you have implemented the solveIrreducibleBoard method, you should do a partial implementation of solve that can solve an easy-level puzzle that never encounters an irreducible board. At that point, the Solve button may return a message saying that the board is irreducible.
Generating Solvable Puzzles
...