class Zoo:
    def __init__(self, grid):
        """
        Initialize a Zoo with the given grid. The grid is a list of list of string.
        Elements are one of: "elephant", "mongoose", "mouse", "snake", None
        """
        self.grid = grid

    def is_valid(self):
        """
        Returns True if the Zoo is valid
        """
        
        raise ValueError("implement me")