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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 ...XIn the above board there are 2 battleships.

**Invalid Example:**

...X XXXX ...XThis 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?

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