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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def in_order_traverse(self):
 """ In-order traversal of the tree """

        result = []

        if not self.node:
            return result

        result.extend(self.node.left.in_order_traverse())
        result.append(self.node.key)
        result.extend(self.node.right.in_order_traverse())
        return result