输入元素，构造平衡二叉树

class TreeNode:
    def __init__(self, val=None):
        self.val = val
        self.left = None
        self.right = None


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

    def insert(self, val):
        self.root = self._insert(self.root, val)

    def _insert(self, node, val):
        if not node:
            return TreeNode(val)
        if val < node.val:
            node.left = self._insert(node.left, val)
        elif val > node.val:
            node.right = self._insert(node.right, val)
        else:
            return node

        # 计算节点的平衡因子
        balance = self._get_balance(node)

        # LL型不平衡
        if balance > 1 and val < node.left.val:
            return self._rotate_right(node)

        # RR型不平衡
        if balance < -1 and val > node.right.val:
            return self._rotate_left(node)

        # LR型不平衡
        if balance > 1 and val > node.left.val:
            node.left = self._rotate_left(node.left)
            return self._rotate_right(node)

        # RL型不平衡
        if balance < -1 and val < node.right.val:
            node.right = self._rotate_right(node.right)
            return self._rotate_left(node)

        return node

    def _get_height(self, node):
        if not node:
            return -1
        return 1 + max(self._get_height(node.left), self._get_height(node.right))

    def _get_balance