Validate Binary Search Tree

validate_bst.py

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

class Solution:
    def isValidBST(self, root: TreeNode) -> bool:
        def helper(tree, mi, mx):
            if not tree:
                return True
            if not mi < tree.val < mx:
                return False
            return helper(tree.left, mi, tree.val) and helper(tree.right, tree.val, mx)
        
        return helper(root, float('-inf'), float('inf'))