[LeetCode] 110. Balanced Binary Tree (Python)

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# Definition for a binary tree node.
from typing import Optional


class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right


class Solution:
    def isBalanced(self, root: Optional[TreeNode]) -> bool:
        def dfs(node) -> int:
            if not node:
                return 0

            left_height = dfs(node.left)
            right_height = dfs(node.right)

            if left_height == -1 or right_height == -1 \
                    or abs(left_height - right_height) > 1:
                return -1

            return max(left_height, right_height) + 1

        return dfs(root) >= 0