<p>Given an <code>m x n</code> matrix <code>board</code> where each cell is a battleship <code>'X'</code> or empty <code>'.'</code>, return <em>the number of the <strong>battleships</strong> on</em> <code>board</code>.</p> <p><strong>Battleships</strong> can only be placed horizontally or vertically on <code>board</code>. In other words, they can only be made of the shape <code>1 x k</code> (<code>1</code> row, <code>k</code> columns) or <code>k x 1</code> (<code>k</code> rows, <code>1</code> column), where <code>k</code> can be of any size. At least one horizontal or vertical cell separates between two battleships (i.e., there are no adjacent battleships).</p> <p> </p> <p><strong class="example">Example 1:</strong></p> <img alt="" src="https://assets.leetcode.com/uploads/2021/04/10/battelship-grid.jpg" style="width: 333px; height: 333px;" /> <pre> <strong>Input:</strong> board = [["X",".",".","X"],[".",".",".","X"],[".",".",".","X"]] <strong>Output:</strong> 2 </pre> <p><strong class="example">Example 2:</strong></p> <pre> <strong>Input:</strong> board = [["."]] <strong>Output:</strong> 0 </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>m == board.length</code></li> <li><code>n == board[i].length</code></li> <li><code>1 <= m, n <= 200</code></li> <li><code>board[i][j]</code> is either <code>'.'</code> or <code>'X'</code>.</li> </ul> <p> </p> <p><strong>Follow up:</strong> Could you do it in one-pass, using only <code>O(1)</code> extra memory and without modifying the values <code>board</code>?</p>