NumberOfIslandsBFS.java

public class Block {
    public int x;
    public int y;

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

public int numIslandsBFS(char[][] grid) {
    if (grid == null || grid.length == 0 || grid[0].length == 0) {
        return 0;
    }

    int m = grid.length;
    int n = grid[0].length;

    boolean[][] visited = new boolean[m][n];
    int count = 0;

    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++) {
            if (grid[i][j] == '0' || visited[i][j]) {
                continue;
            }

            count++;
            visited[i][j] = true;

            Queue<Block> q = new LinkedList<>(); // queue just uses LinkedList, ArrayList doesn't implement it

            // remind myself what's the diff between offer and add, 
            // some implementation it's the same, some add throws while offer return true/false if the queue is
            // full while you still trying to add
            q.offer(new Block(i, j)); 

            while (!q.isEmpty()) {
                Block b = q.poll();

                for (int k = 0; k < 4; k++) {
                    int x = b.x + xDirection[k];
                    int y = b.y + yDirection[k];
                    if (this.shouldExplore(x, y, m, n, grid, visited)) {
                        visited[x][y] = true;
                        q.offer(new Block(x, y));
                    }
                }
            }
        }
    }
    return count;
}

private boolean shouldExplore(int x, int y, int row, int col, char[][] grid, boolean[][] visited) {
    if (x >= 0 && x < row && y >= 0 && y < col && grid[x][y] == '1' && !visited[x][y]) return true;

    return false;
}