417. Pacific Atlantic Water Flow

class Solution {
    public List<int[]> pacificAtlantic(int[][] matrix) {
        List<int[]> list = new ArrayList<>();
        if(matrix == null || matrix.length < 1 || matrix[0].length < 1) return list;
        int m = matrix.length, n = matrix[0].length;
        int[][] f = new int[m][n];
        for(int i = 0; i < m; i++)
        {
            fill(matrix, i, 0, f, 0);
            fill(matrix, i, n - 1, f, 1);
        }
        for(int j = 0; j < n; j++)
        {
            fill(matrix, 0, j, f, 0);
            fill(matrix, m - 1, j, f, 1);
        }

        for(int i = 0; i < m; i++)
        {
            for(int j = 0; j < n; j++)
            {
                if(f[i][j] == 3) list.add(new int[]{i, j});

            }
        }
        return list;
    }


    void fill(int[][] matrix, int i, int j, int[][] f, int source)
    {
        int m = matrix.length, n = matrix[0].length, inf = Integer.MAX_VALUE;
        int[] dx = {1, 0, -1, 0}, dy = {0, 1, 0, -1};
        if((f[i][j] & (1 << source)) != 0) return;
        f[i][j] |= 1 << source;
        for(int k = 0; k < 4; k++)
        {
            int ni = i + dx[k], nj = j + dy[k];
            if(ni < 0 || nj < 0 || ni >= m || nj >= n || matrix[ni][nj] < matrix[i][j]) continue;
            fill(matrix, ni, nj, f, source);
        }
    }
}