Just Combine Valid Sudoku and N-Queens

the only difference form N-Queens is the possibility checker.

but, good news is that we have done Valid Sudoku.

Time Limit

The first version looks like N-Queens may exceed the time limit.

Fail fast.

In this problem, if any grid is not solved, trying any further is needless.

So, let solveSudoku return a flag to warn this way is blocked.