보드 카운트의 전함

var countBattleships = function(board) {

    if (board === null || board.length === 0 || board[0].length == 0 ) 
        return 0;

    let res = 0;

    const m = board.length;
    const n = board[0].length;

    for(let i = 0; i < m; i++) {
        for (let j = 0; j < n; j++) {
            if (board[i][j] === '.' 
                || 
                    (i > 0 && board[i - 1][j] === 'X') 
                || 
                    (j > 0 && board[i][j - 1] === 'X')
               ) 
                continue;
            res++;
        }
    }

    return res;
};