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# Runtime: 89 ms (Top 18.35%) | Memory: 16.5 MB (Top 80.77%)
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# Definition for a binary tree node.
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# class TreeNode:
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# def __init__(self, val=0, left=None, right=None):
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# self.val = val
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# self.left = left
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# self.right = right
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class Solution:
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    def isValidBST(self, root: Optional[TreeNode]) -> bool:
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        def valid(node, left, right):
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            if not node:  # checking node is none
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                return True
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            if not (
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                node.val > left and node.val < right
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            ):  # checking the left value is less than node and right value is greater than node
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                return False
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            return valid(node.left, left, node.val) and valid(
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                node.right, node.val, right
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            )  # recursively calling left child and right child and returing the result True if both are true else False
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        return valid(
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            root, float("-inf"), float("inf")
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        )  # calling recursive function to check