Balanced Binary Tree

......
2    def __init__(self, val=0, left=None, right=None):
3        self.val = val
4        self.left = left
5        self.right = right
6 
7class Solution:
●    def isBalanced(self, root) -> bool:
9        def check(node):
10            if not node:          # null leaf has height 0
11                return 0
12 
13            left = check(node.left)
14            if left == -1:        # early exit - left unbalanced
......