leetcode417. Pacific Atlantic Water Flow

Given an m x n matrix of non-negative integers representing the height of each unit cell in a continent, the "Pacific ocean" touches the left and top edges of the matrix and the "Atlantic ocean" touches the right and bottom edges.

Water can only flow in four directions (up, down, left, or right) from a cell to another one with height equal or lower.

Find the list of grid coordinates where water can flow to both the Pacific and Atlantic ocean.

Note:
The order of returned grid coordinates does not matter.
Both m and n are less than 150.
Example:

Given the following 5x5 matrix:

  Pacific ~   ~   ~   ~   ~ 
       ~  1   2   2   3  (5) *
       ~  3   2   3  (4) (4) *
       ~  2   4  (5)  3   1  *
       ~ (6) (7)  1   4   5  *
       ~ (5)  1   1   2   4  *
          *   *   *   *   * Atlantic

Return:

[[0, 4], [1, 3], [1, 4], [2, 2], [3, 0], [3, 1], [4, 0]] (positions with parentheses in above matrix).

{1,2,3},
{8,9,4},
{7,6,5}

    public List<int[]> pacificAtlantic(int[][] matrix) {
        List<int[]> result = new ArrayList<>();
        if(matrix==null || matrix.length==0 || matrix[0].length==0) return result;
        int rows = matrix.length;
        int columns = matrix[0].length;
        //能够流入太平洋
        boolean[][] canReachPacific = new boolean[matrix.length][matrix[0].length];
        //能够流入大西洋
        boolean[][] canReachAtlantic = new boolean[matrix.length][matrix[0].length];
        for(int i = 0 ; i<rows ; i++) {
            //以左侧边为起点寻找可以流入太平洋的所有节点
            dfs(matrix, canReachPacific, i, 0);
            //以右侧边为起点寻找可以流入大西洋的所有节点
            dfs(matrix, canReachAtlantic, i, matrix[0].length-1);
        }
        
        for(int i = 0 ; i<columns ; i++) {
            //以上侧边为起点寻找可以流入太平的所有节点
            dfs(matrix, canReachPacific, 0, i);
            //以下侧边为起点寻找可以流入大西洋的所有节点
            dfs(matrix, canReachAtlantic, matrix.length-1,  i);
        }
        
        for(int i = 0 ; i<rows ; i++) {
            for(int j = 0 ; j<columns ; j++) {
                //将即可以流入太平洋也可以流入大西洋的水域加入结果集
                if(canReachPacific[i][j] && canReachAtlantic[i][j]) {
                    result.add(new int[]{
                            i, j
                    });
                }
            }
        }
        return result;
    }