Solution.java

class Solution {
    private String[] board;

    public boolean validTicTacToe(String[] board) {
        this.board = board;
        int x = count('X'), o = count('O');
        if (x != o && x - 1 != o) {
            return false;
        }
        if (win('X') && x - 1 != o) {
            return false;
        }
        return !(win('O') && x != o);
    }

    private boolean win(char x) {
        for (int i = 0; i < 3; ++i) {
            if (board[i].charAt(0) == x && board[i].charAt(1) == x && board[i].charAt(2) == x) {
                return true;
            }
            if (board[0].charAt(i) == x && board[1].charAt(i) == x && board[2].charAt(i) == x) {
                return true;
            }
        }
        if (board[0].charAt(0) == x && board[1].charAt(1) == x && board[2].charAt(2) == x) {
            return true;
        }
        return board[0].charAt(2) == x && board[1].charAt(1) == x && board[2].charAt(0) == x;
    }

    private int count(char x) {
        int cnt = 0;
        for (var row : board) {
            for (var c : row.toCharArray()) {
                if (c == x) {
                    ++cnt;
                }
            }
        }
        return cnt;
    }
}