Tic-tac-toe or noughts and crosses is a childhood game where players take turns playing X's and O's on a 3x3 board.

TicTacToe.java

import java.util.Scanner;

/**
 * Tic-tac-toe or noughts and crosses is a childhood game where players take turns playing X's and O's
 * on a 3x3 board trying to get three in a row. Here's an example game where O is victorious:
 * <p>
 * Write a function that takes an NxN game board as input and returns true or false depending on whether it's a valid board.
 * The valid states for squares of the board are "X", "O", and "~" (no one has played there yet).<br>
 * Examples of valid boards:
 * <ol style="margin-top: 0">
 *    <li>A board with a game in progress</li>
 *    <li> A board where there is a winner.</li>
 * </ol>
 * <p>
 * Examples of invalid boards:
 * <ol style="margin-top: 0">
 *     <li>One where one player has played more than another player (excluding the case where X has played, and it's O's turn)</li>
 * </ol>
 * NOTE: In our variation of the game, X is always the first player to play.
 * <p>
 * INPUT
 * <ul style="margin-top: 0">
 *     <li>Line 1: N the size of the board</li>
 *     <li>Line 2: N states of the first line</li>
 *     <li>...</li>
 *     <li>Line N+1: N states of the Nth line</li>
 * </ul>
 * <p>
 * OUTPUT:
 * True if the board is valid otherwise False.
 * <p>
 * CONSTRAINTS:
 * 0 < N < 1000
 * <p>
 * EXAMPLE:
 * <table border="1" style="border: 1px solid black; border-collapse: collapse;">
 *     <tr>
 *         <td>Input</td>
 *         <td>Output</td>
 *     </tr>
 *
 *     <tr>
 *         <td>3</td>
 *         <td>True</td>
 *     </tr>
 *     <tr>
 *         <td>XOO</td>
 *     </tr>
 *     <tr>
 *         <td>OXX</td>
 *     </tr>
 *     <tr>
 *         <td>OXX</td>
 *     </tr>
 * </table>
 */
public class TicTacToe {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        if (in.hasNextLine()) {
            in.nextLine();
        }

        int xPlays = 0, oPlays = 0;
        String[] board = new String[n];
        for (int i = 0; i < n; i++) {
            String line = in.nextLine();

            // store
            board[i] = line;
            // process
            for (int idx = 0; idx < line.length(); idx++) {
                if (line.charAt(idx) == 'O') {
                    oPlays++;
                } else if (line.charAt(idx) == 'X') {
                    xPlays++;
                }
            }
        }

        // Check
        if (xPlays >= 1 && board[0].charAt(0) == 'X' // X is always the first player to play.
            && (
                    xPlays == oPlays ||
                    (xPlays > oPlays && oPlays + 1 == xPlays) ||
                    (oPlays > xPlays && xPlays + 1 == oPlays)
            )
        ) {
            System.out.println("True");
        } else {
            System.out.println("False");
        }
    }
}