794. Valid Tic-Tac-Toe State

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"}.

# @lc code=start
using LeetCode

function validTicTacToe(board::Array{Char})::Bool
    n_X, n_O = 0, 0

    for x in board
        if x == 'X'
            n_X += 1
        elseif x == 'O'
            n_O += 1
        end
    end

    if n_O > n_X || n_X >= n_O + 2
        false
    elseif n_X >= 3
        if n_X == n_O
            if is_win(board, 'X')
                false  ## Impossible! n_X should == x_O + 1
            else
                true
            end
        else
            # @assert n_X == n_O + 1
            if is_win(board, 'O')
                false  ## Impossible!
            else
                true
            end
        end
    else
        true
    end
end

function is_win(b, x)
    return b[1] == b[2] == b[3] == x ||
           b[4] == b[5] == b[6] == x ||
           b[7] == b[8] == b[9] == x ||
           b[1] == b[4] == b[7] == x ||
           b[2] == b[5] == b[8] == x ||
           b[3] == b[6] == b[9] == x ||
           b[1] == b[5] == b[9] == x ||
           b[3] == b[5] == b[7] == x
end
# @lc code=end

is_win (generic function with 1 method)

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