The game Sudoku involves placing numbers in a 9 x 9 grid in such a way that each number from 1 through 9 appears exxactly once in each row, once in each column, and once in each of certain 3x3 areas on the board. You are to write a class called NineChecker that would be useful in checking a solution to Sudoku. A NineChecker object holds an array of nine booleans, and has the following methods:
public void mark(int number) is used to record the fact that a number has been seen. It does so by setting the corresponding element of the array to true. You can assume number is from 1 to 9. (Be careful: the array indices run from 0 to 8.)
public boolean allFound( ) returns true if all the elements of the array are true, otherwise it returns false.