419. Battleships in a Board

Given an 2D board, count how many battleships are in it. The battleships are represented with 'X's, empty slots are represented with '.'s. You may assume the following rules:

• 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 an invalid board that you will not receive - as battleships will always have a cell separating between them.

Follow up: Could you do it in one-pass , using only O(1) extra memory and without modifying the value of the board?

# @lc code=start
using LeetCode

function count_battleships(board::Matrix{Char})
    res = 0
    for I in CartesianIndices(board)
        board[I] == '.' && continue
        i, j = I.I
        (i == 1 || board[i - 1, j] == '.') &&
            (j == 1 || board[i, j - 1] == '.') &&
            (res += 1)
    end
    return res
end
# @lc code=end

count_battleships (generic function with 1 method)

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