Trapping rain water II

public class Solution {
    /**
     * @param heights: a matrix of integers
     * @return: an integer
     */

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

    public int trapRainWater(int[][] heights) {

        // null check
        if(heights == null || heights.length == 0)  
            return 0;

        // min heap    
        Queue<Cell> pq = new PriorityQueue<Cell>(1, new Comparator<Cell>() {
            public int compare(Cell c1, Cell c2) {
                return c1.h - c2.h;
            }
        });



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

        // record if visited
        boolean[][] visited = new boolean[m][n];

        // add all cells at edage into pq
        for (int i = 0; i < m; i++) {
            pq.offer(new Cell(i,0,heights[i][0]));
            pq.offer(new Cell(i,n-1,heights[i][n-1]));
            visited[i][0] = true;
            visited[i][n-1] = true;
        }

        for (int i = 0; i < n; i++) {
            pq.offer(new Cell(0,i,heights[0][i]));
            pq.offer(new Cell(m-1,i,heights[m-1][i]));
            visited[0][i] = true;
            visited[m-1][i] = true;
        }


        int res = 0 ;

        // use pq to find min height each time
        // and then get water for at most 4 cells for cur cell
        while(!pq.isEmpty()) {
            Cell now = pq.poll();

            for(int i = 0; i < 4; i++) {
                int nx = now.x + dx[i];
                int ny = now.y + dy[i];

                if(0 <= nx && nx < m && 0 <= ny && ny < n && !visited[nx][ny]) {
                    visited[nx][ny] = true;
                    pq.offer(new Cell(nx,ny,Math.max(now.h,heights[nx][ny])));
                    res += Math.max(0, now.h - heights[nx][ny]);
                }
            }   
        }

        return res;

    }


    // needs a DS 

    class Cell {
        int x, y, h;

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