python leetcode 417. Pacific Atlantic Water Flow

考察DFS， 难点在于要分别判断是否能到达两大洋及如何设置搜索起始位置

class Solution:
    def pacificAtlantic(self, matrix):
        """
        :type matrix: List[List[int]]
        :rtype: List[List[int]]
        """
        m=len(matrix)
        if m==0: return []
        n=len(matrix[0])
        Pdp=[[False]*n for _ in range(m)]
        Adp=[[False]*n for _ in range(m)]
        for i in range(m):
            self.dfs(Pdp,matrix,m,n,i,0)
            self.dfs(Adp,matrix,m,n,i,n-1)
        for j in range(n):
            self.dfs(Pdp,matrix,m,n,0,j)
            self.dfs(Adp,matrix,m,n,m-1,j)
        res=[]
        for i in range(m):
            for j in range(n):
                if Pdp[i][j] and Adp[i][j]:
                    res.append([i,j])
        return res
    def dfs(self,visit,matrix,m,n,x,y):
        visit[x][y]=True 
        for dx,dy in zip((-1,0,1,0),(0,-1,0,1)):
            nx,ny=x+dx,y+dy
            if nx<0 or nx>=m or ny<0 or ny>=n or visit[nx][ny] or matrix[nx][ny]<matrix[x][y]:
                continue 
            self.dfs(visit,matrix,m,n,nx,ny)