public class Solution {

    /**

     * @param grid: a boolean 2D matrix

     * @return: an integer

     */

    public int numIslands(boolean[][] grid) {

      if (grid == null || grid.length == 0 || grid[0].length == 0){

          return 0;

      }

      boolean[][] isVisited = new boolean[grid.length][grid[0].length];

      int result = 0;

      for (int i = 0; i < grid.length; i++){

           for (int j = 0; j < grid[0].length; j++){

                 if (grid[i][j] && !isVisited[i][j]){

                    result++;

                    bfs(grid, isVisited, i, j);

                }

           }

      }

     return result;

    }

    private void bfs(boolean[][] grid, boolean[][] isVisited, int startx, int endy){

        //direction

        int[] dx = {0, 0, 1, -1};

        int[] dy = {1, -1, 0, 0};

        // 記錄 x, y

        Queue<Integer> qx = new LinkedList<>();

        Queue<Integer> qy = new LinkedList<>();

        // 入隊列

        qx.offer(startx);

        qy.offer(endy);

        isVisited[startx][endy] = true;

        // 隊列不為空，不停的從隊列裡拿出一個點，拓展鄰居節點放到隊列中

        while (!qx.isEmpty()){

            // 從隊列取出一個點

            int curx = qx.poll();

            int cury = qy.poll();

            // 展開

            for (int i = 0; i < 4; i++){

                int nextx = curx + dx[i];

                int nexty = cury + dy[i];

                if (nextx >= 0 && nextx < grid.length &&

                    nexty >= 0 && nexty < grid[0].length &&

                    !isVisited[nextx][nexty] && grid[nextx][nexty]) {

                        qx.offer(nextx);

                        qy.offer(nexty);

                        isVisited[nextx][nexty] = true;

                    }

            }

        }

    }

}```