419. Battleships in a Board

• Difficulty: Medium

• Topics:

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Problem:

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

Invalid Example:

...X
XXXX
...X

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

Solutions:

class Solution {
public:
    int countBattleships(vector<vector<char>>& board) {
        int count = 0;
        int m = board.size();
        if (m == 0) return 0;
        int n = board[0].size();
        if (n == 0) return 0;

        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (board[i][j] == 'X' && (j - 1 < 0 || board[i][j-1] != 'X') && ( i - 1 < 0 || board[i-1][j] != 'X'))  ++count;
            }
        }

        return count;
    }
};