平衡二叉树 #

给定一个二叉树，判断它是否是高度平衡的二叉树。 本题中，一棵高度平衡二叉树定义为： 一个二叉树每个节点   的左右两个子树的高度差的绝对值不超过 1。 示例 1: ``` 给定二叉树 [3,9,20,null,null,15,7] 3 / \ 9 20 / \ 15 7 ``` 返回 true 。 示例 2: ``` 给定二叉树 [1,2,2,3,3,null,null,4,4] 1 / \ 2 2 / \ 3 3 / \ 4 4 返回 false 。 ```

Given a binary tree, determine if it is height-balanced. For this problem, a height-balanced binary tree is defined as: a binary tree in which the left and right subtrees of every node differ in height by no more than 1. Example 1: ``` Given the following tree [3,9,20,null,null,15,7]: 3 / \ 9 20 / \ 15 7 ``` Return true. Example 2: ``` Given the following tree [1,2,2,3,3,null,null,4,4]: 1 / \ 2 2 / \ 3 3 / \ 4 4 Return false. ```

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```python class Solution: def isBalanced(self, root: TreeNode) -> bool: return self.height(root) >= 0 def height(self, root: TreeNode) -> int: if root is None: return 0 lh = self.height(root.left) rh = self.height(root.right) if lh >= 0 and rh >=0 and abs(lh - rh) <= 1: return max(lh, rh) + 1 return -1 ```

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