func countBattleships(_ board: [[Character]]) -> Int {
    if board.isEmpty || board[0].isEmpty {
        return 0
    }
    var answer = 0
    for i in stride(from: 0, to: board.count, by: 1) {
        for j in stride(from: 0, to: board[0].count, by: 1) {
            if board[i][j] == "." {
                continue
            }
            if i > 0 && board[i-1][j] == "X" {
                continue
            }
            if j > 0 && board[i][j-1] == "X" {
                continue
            }
            answer += 1
        }
    }
    return answer
}