NumberOfIslands.java

class Coordinate {
    int x, y;
    public Coordinate(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

public class Solution {
    /*
     * @param grid: a boolean 2D matrix
     * @return: an integer
     */
    public int numIslands(boolean[][] grid) {
        // write your code here
        if(grid == null || grid.length == 0 || grid[0].length == 0) {
            return 0;
        }
        
        int n = grid.length;
        int m = grid[0].length;
        int islands = 0;
        
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                if (grid[i][j]) {
                    markByBFS(grid, i, j);
                    islands++;
                }
            }
        }
        
        return islands;
        
    }
    
    private void markByBFS(boolean[][] grid, int x, int y) {
        // magic numbers!
        int[] directionX = {0, 1, -1, 0};
        int[] directionY = {1, 0, 0, -1};
        
        Queue<Coordinate> queue = new LinkedList<>();
        
        queue.offer(new Coordinate(x, y));
        grid[x][y] = false;
        
        while (!queue.isEmpty()) {
            Coordinate coor = queue.poll();
            for (int i = 0; i < 4; i++) {
                Coordinate adj = new Coordinate(
                    coor.x + directionX[i],
                    coor.y + directionY[i]
                );
                if (!inBound(adj, grid)) {
                    continue;
                }
                if (grid[adj.x][adj.y]) {
                    grid[adj.x][adj.y] = false;
                    queue.offer(adj);
                }
            }
        }
    }
    
    private boolean inBound(Coordinate coor, boolean[][] grid) {
        int n = grid.length;
        int m = grid[0].length;
        
        return coor.x >= 0 && coor.x < n && coor.y >= 0 && coor.y < m;
    }
}