平衡二叉树：二叉树中任意节点的左右子树的深度相差不超过1，那就该二叉树就是一颗平衡二叉树。

    如何求一颗二叉树的高度/深度?

    1. # Definition for a binary tree node.
    2. # class TreeNode:
    3. #     def __init__(self, x):
    4. #         self.val = x
    5. #         self.left = None
    6. #         self.right = None
    8. class Solution:
    9.     def treeDepth(self, root: TreeNode) -> bool:
    10.         if root is None:
    11.             return 0
    12.         else:
    13.             return max(self.treeDepth(root.left), self.treeDepth(root.right)) + 1

    image.png

    https://leetcode-cn.com/problems/ping-heng-er-cha-shu-lcof/solution/mian-shi-ti-55-ii-ping-heng-er-cha-shu-cong-di-zhi/

    1. # Definition for a binary tree node.
    2. # class TreeNode:
    3. #     def __init__(self, x):
    4. #         self.val = x
    5. #         self.left = None
    6. #         self.right = None
    8. class Solution:
    9.     def isBalanced(self, root: TreeNode) -> bool:
    10.         def recur(root):
    11.             if not root:
    12.                 return 0
    13.             left = recur(root.left)
    14.             if left == -1:
    15.                 return -1
    16.             right = recur(root.right)
    17.             if right == -1:
    18.                 return -1
    19.             return max(left, right) + 1 if abs(left - right) <= 1 else -1
    20.         return recur(root) != -1