Validate Binary Search Tree Leetcode Python

Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a BST is defined as follows:
The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.
定義に基づいてhttp://en.wikipedia.org/wiki/Binary_search_tree
The left subtree of a node contains only nodes with keys less than the node's key. The right subtree of a node contains only nodes with keys greater than the node's key. The left and right subtree each must also be a binary search tree. Each node can have up to two successor nodes. There must be no duplicate nodes. A unique path exists from the root to every other node.
左サブツリーはノードが右サブツリーより小さく、重複する値はありません.左右のノードとルートノードの値を比較するたびに、1つのノードが条件を満たさない限りFALSEに戻ります.
we compare the left, right node key with root key. If any of the condition cannot match the requirement, we return False.

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return a boolean
    def testnode(self,root,minval,maxval):
        if root==None:
            return True
        if root.val>maxval or root.val<minval:
            return False
        return self.testnode(root.left,minval,root.val-1) and self.testnode(root.right,root.val+1,maxval)
    def isValidBST(self, root):
        
        minval=-3147483648
        maxval=3147483648
        return self.testnode(root,minval,maxval)