Consider the task of writing a program that plays TicTacToe against a human opponent. A user interface TicTacToeUI reads the user's moves and displays the computer's moves and the board. A class TicTacToeStrategy determines the next move that the computer makes. A class TicTacToeBoard represents the current state of the board. Complete all classes except for the strategy class. Instead, use a mock class that simply picks the first available empty square.