98. Validate Binary Search Tree

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1: return True, 3: return True. 2: return True

# 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: Optional[TreeNode]) - bool:
 def validate(root, low, high):
 if not root: return True
 if root.val low or root.val high:
 return False
 return validate(root.left, low, root.val) and validate(root.right, root.val, high)
 return validate(root, -sys.maxsize, sys.maxsize)