419. Battleships in a Board (Medium)

https://leetcode.com/problems/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?

Hints

Solutions

  • Java
class Solution {
    public int countBattleships(char[][] board) {
        int count = 0;
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                if (board[i][j] != 'X') {
                    continue;
                }

                if (i - 1 >= 0 && board[i - 1][j] == 'X') { //check for the boundary condition
                        continue;
                }

                if (j - 1 >= 0 && board[i][j - 1] == 'X') {
                        continue;
                }

                count++;
            }
        }

        return count;
    }
}
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