AVL 树是平衡二叉搜索树，所以首先 必须满足搜索树，即l_child值<parent值<=r_child值。这个在前面已经实现了。插入和删除不停的递归就可以。-Two AVL tree is balanced binary search trees, so we must first meet the search tree, l_child value and <parent value of <=r_child value. The in front has been achieved. Insert and delete the recursive kept can.