ValidTicTacToeState.java

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// A Tic-Tac-Toe board is given as a string array board. Return True if and only if it is possible to reach this board position during the course of a valid tic-tac-toe game.

// The board is a 3 x 3 array, and consists of characters " ", "X", and "O".  The " " character represents an empty square.

// Here are the rules of Tic-Tac-Toe:

// Players take turns placing characters into empty squares (" ").
// The first player always places "X" characters, while the second player always places "O" characters.
// "X" and "O" characters are always placed into empty squares, never filled ones.
// The game ends when there are 3 of the same (non-empty) character filling any row, column, or diagonal.
// The game also ends if all squares are non-empty.
// No more moves can be played if the game is over.
// Example 1:
// Input: board = ["O  ", "   ", "   "]
// Output: false
// Explanation: The first player always plays "X".

// Example 2:
// Input: board = ["XOX", " X ", "   "]
// Output: false
// Explanation: Players take turns making moves.

// Example 3:
// Input: board = ["XXX", "   ", "OOO"]
// Output: false

// Example 4:
// Input: board = ["XOX", "O O", "XOX"]
// Output: true
// Note:

// board is a length-3 array of strings, where each string board[i] has length 3.
// Each board[i][j] is a character in the set {" ", "X", "O"}.

class Solution {
    private char[][] gameBoard = new char[3][3]; 

    public boolean validTicTacToe(String[] board) {
        
        unpackBoard(board);

        return gameBoardValidation();
    }

    private void unpackBoard(String[] board){
        for(int i=0; i<3;i++){
            for(int j=0; j<3; j++){
                this.gameBoard[i][j] = board[i].charAt(j);
            }
        }
    }

    private boolean gameBoardValidation(){
        
        int numX, numO;
        
        numX = count('X');
        numO = count('O');
        
        //X goes first
        if(numO>numX) return false;
        
        //players take turns
        if(numX>numO+1) return false;
        
        //both players can't win
        if(winner('X') && winner('O')) return false;
        
        //game ends when one player wins
        if(winner('X') && numX==numO) return false;
        
        //game ends when one player wins
        if(winner('O') && numX>numO) return false;
        
        return true;
    }

    private int count(char player){
        
        int num = 0;
        
        for(int i=0; i<3;i++){
            for(int j=0; j<3; j++){
                if(gameBoard[i][j] == player) num++;
            }
        }
        
        return num;
    }

    private boolean winner(char player){
        
        if(validateRows(player)) return true;
        if(validateColumns(player)) return true;
        if(validateDiagonal(player)) return true;
        
        return false;
    }

    private boolean validateRows(char player){
        for(int i=0; i<3; i++){
            if(gameBoard[i][0] == player && gameBoard[i][1] == player && gameBoard[i][2] == player) return true;
        }
        
        return false;
    }

    private boolean validateColumns(char player){
        for(int i=0; i<3; i++){
            if(gameBoard[0][i] == player && gameBoard[1][i] == player && gameBoard[2][i] == player) return true;
        }
        
        return false;
    }

    private boolean validateDiagonal(char player){
        if(gameBoard[0][0] == player && gameBoard[1][1] == player && gameBoard[2][2] == player) return true;
        if(gameBoard[0][2] == player && gameBoard[1][1] == player && gameBoard[2][0] == player) return true;
        
        return false;
    }
}