Python simple and clean recursive solution


  • 2
    # Definition for a binary tree node.
    # class TreeNode(object):
    #     def __init__(self, x):
    #         self.val = x
    #         self.left = None
    #         self.right = None
    
    class Solution(object):
        def isValidBST(self, root):
            """
            :type root: TreeNode
            :rtype: bool
            """
            def valid(node, lower, upper):
                if not node:
                    return True
                if lower is not None and node.val <= lower:
                    return False
                if upper is not None and node.val >= upper:
                    return False
                return valid(node.left, lower, node.val) and valid(node.right, node.val, upper)
            return valid(root, None, None)
    

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