I misunderstood the description. It actually says that all battleships on the given board are 1 x k or k x 1, and it doesn’t mean that we need to find those single row/col ourselves.

So instead of dfs, just count the start of each battleship.

Question:

Given an m x n matrix board where each cell is a battleship 'X' or empty '.', return the number of the battleships on board.

Battleships can only be placed horizontally or vertically on board. In other words, they can only be made of the shape 1 x k (1 row, k columns) or k x 1 (k rows, 1 column), where k can be of any size. At least one horizontal or vertical cell separates between two battleships (i.e., there are no adjacent battleships).