平衡二叉树，又称AVL树。它或者是一棵空树，或者是具有下列性质的二叉树：它的左子树和右子树都是平衡二叉树，且左子树和右子树的高度之差之差的绝对值不超过1.。

-Balanced binary tree, also known as AVL tree. It is either an empty tree, or a binary tree with the following properties: it left sub-tree and right sub-trees are balanced binary tree, and the left subtree and right sub-tree height difference between the absolute difference between the value does not exceed 1..