98. Validate Binary Search Tree - LeetCode

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def isValidBST(self, root, lo=float('-inf'), hi=float('inf')) -> bool:
        '''
        [5,4,6,null,null,3,7] => false
        '''
        return (not root) or all(
            [
                lo < root.val < hi,
                self.isValidBST(root.left, lo, root.val),
                self.isValidBST(root.right, root.val, hi),
            ]
        )