AVL 树是一种常见的数据结构，所谓的二叉平衡树，这里是一个简单实现，算法效率可能并不太高，因为只是简单实现，方便大家学习使用-AVL tree is a common data structure, the so-called binary balanced tree, here is a simple implementation of the efficiency of the algorithm may not be too high, because only a simple implementation to facilitate them to learn to use