Solution.js

/**
 * @param {string[]} board
 * @return {boolean}
 */
var validTicTacToe = function (board) {
    function count(x) {
        let cnt = 0;
        for (const row of board) {
            for (const c of row) {
                cnt += c == x;
            }
        }
        return cnt;
    }
    function win(x) {
        for (let i = 0; i < 3; ++i) {
            if (board[i][0] == x && board[i][1] == x && board[i][2] == x) {
                return true;
            }
            if (board[0][i] == x && board[1][i] == x && board[2][i] == x) {
                return true;
            }
        }
        if (board[0][0] == x && board[1][1] == x && board[2][2] == x) {
            return true;
        }
        return board[0][2] == x && board[1][1] == x && board[2][0] == x;
    }
    const [x, o] = [count('X'), count('O')];
    if (x != o && x - 1 != o) {
        return false;
    }
    if (win('X') && x - 1 != o) {
        return false;
    }
    return !(win('O') && x != o);
};