417. Pacific Atlantic Water Flow

depth first search solution 
class Solution(object):
    def pacificAtlantic(self, matrix):
        """
        :type matrix: List[List[int]]
        :rtype: List[List[int]]
        """
        if not matrix: return []
        self.directions = [(1,0),(-1,0),(0,1),(0,-1)]
        m = len(matrix)
        n = len(matrix[0])
        p_visited = [[False for _ in range(n)] for _ in range(m)]
        a_visited = [[False for _ in range(n)] for _ in range(m)]
        result = []
        
        for i in range(m):
            
            self.dfs(matrix, i, 0, p_visited, m, n)
            self.dfs(matrix, i, n-1, a_visited, m, n)
        for j in range(n):
           
            self.dfs(matrix, 0, j, p_visited, m, n)
            self.dfs(matrix, m-1, j, a_visited, m, n)
        
        for i in range(m):
            for j in range(n):
                if p_visited[i][j] and a_visited[i][j]:
                    result.append([i,j])
        return result
                
                
    def dfs(self, matrix, i, j, visited, m, n):
        # when dfs called, meaning its caller already verified this point 
        #print 'visited',i,j
        visited[i][j] = True
        for dir in self.directions:
            x, y = i + dir[0], j + dir[1]
            #print 'trying to visit', x,y
            if x < 0 or x >= m or y < 0 or y >= n or visited[x][y] or matrix[x][y] < matrix[i][j]:
               
                continue
            self.dfs(matrix, x, y, visited, m, n)