中序遍历 AVL 树可通过递归方式实现。以下是一个示例代码：

class AVLNode:
    def __init__(self, key):
        self.key = key
        self.left = None
        self.right = None
        self.height = 1

class AVLTree:
    def __init__(self):
        self.root = None

    def height(self, node):
        if node is None:
            return 0
        else:
            return node.height

    def balance_factor(self, root):
        if root is None:
            return 0
        return self.height(root.left) - self.height(root.right)

    def left_rotate(self, root):
        y = root.right
        T2 = y.left

        y.left = root
        root.right = T2

        root.height = 1 + max(self.height(root.left), self.height(root.right))
        y.height = 1 + max(self.height(y.left), self.height(y.right))

        return y

    def right_rotate(self, root):
        y = root.left
        T3 = y.right

        y.right = root
        root.left = T3

        root.height = 1 + max(self.height(root.left), self.height(root.right))
        y.height = 1 + max(self.height(y.left), self.height(y.right))

        return y

    def insert(self, root, key):
        if root is None:
            return AVLNode(key)
        elif key < root.key:
            root.left = self.insert(root.left, key)
        else:
            root.right = self.insert(root.right, key)

        root.height = 1 + max(self.height(root.left), self.height(root.right))

        balance = self.balance_factor(root)

        if balance > 1 and key < root.left.key:
            return self.right_rotate(root)
        if balance < -1 and key > root.right.key:
            return self.left_rotate(root)
        if balance > 1 and key > root.left.key:
            root.left = self.left_rotate(root.left)
            return self.right_rotate(root)
        if balance < -1 and key < root.right.key:
            root.right = self.right_rotate(root.right)
            return self.left_rotate(root)

        return root

    def in_order_traversal(self,