leetcode 419.battleships in a board

You receive a valid board, made of only battleships or empty slots.
Battleships can only be placed horizontally or vertically. In other words, they can only be made of the shape 1xN (1 row, N columns) or Nx1 (N rows, 1 column), where N can be of any size.
At least one horizontal or vertical cell separates between two battleships - there are no adjacent battleships.

Example:

X..X
…X
…X
In the above board there are 2 battleships.

Invalid Example:

…X
XXXX
…X
This is not a valid board - as battleships will always have a cell separating between them.

Your algorithm should not modify the value of the board.

####解题思路
由于board中的战舰之间确保有’.’隔开，因此遍历board，若某单元格为’X’，只需判断其左边和上边的相邻单元格是否也是’X’。如果左邻居或者上邻居单元格是’X’，则说明当前单元格是左边或者上边战舰的一部分；否则，令计数器+1

public class Solution {
    public int countBattleships(char[][] board) {
        int len1 = board.length;
        if(len1 == 0){
            return 0;
        }
        int len2 = board[0].length;
        int ans = 0;
        for (int i = 0; i < len1; i++){
            for(int j=0 ; j < len2; j++){
                if(board[i][j] == 'X' && (i == 0 || board[i-1][j] == '.') && (j == 0 || board[i][j-1] == '.')){
                    ans ++;
                }
            }
        }
        return ans;
    }
}