417. Pacific Atlantic Water Flow

class Solution:
    def pacificAtlantic(self, matrix: List[List[int]]) -> List[List[int]]:
        if not matrix or not matrix[0]:
            return []
        r, c = len(matrix), len(matrix[0])
        p_visited = set()
        a_visited = set()
        directions = [(-1, 0), (1, 0), (0, 1), (0, -1)]
        
        def dfs(visited, x, y):
            visited.add((x,y))
            for dx, dy in directions:
                new_x, new_y = x+dx, y+dy
                if 0 <= new_x < r and 0 <= new_y < c and (new_x, new_y) not in visited and matrix[new_x][new_y] >= matrix[x][y]:
                    dfs(visited, new_x, new_y)
        # from left border and right border
        for i in range(r):
            dfs(p_visited, i, 0)
            dfs(a_visited, i, c-1)
        # from up border and bottom border
        for i in range(c):
            dfs(p_visited, 0, i)
            dfs(a_visited, r-1, i)
        return list(p_visited.intersection(a_visited))