1.The inorder traversal sequence of an AVL tree must be in sorted (non-decreasing) order.
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根据二叉搜索树性质，中序遍历一定是有序的，而且是递增的。
2.Insert 1, 2, 3, 4, 5, and 6 one by one into an initially empty AVL tree. Then the preorder traversal sequence of the resulting tree must be {4, 2, 1, 3, 5, 6}.
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最终的AVL树：
3.For any node in an AVL tree, the height of the left subtree must be greater than that of the right subtree.
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4.For an AVL tree, the balance factors of all the non-leaf nodes are 0 iff the tree is a complete binary tree.
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只有完美二叉树满足这个定义。
