```
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def helper(self,root):
if not root:
return 0
leftht = self.helper(root.left)
if leftht == -1:
return -1
rightht = self.helper(root.right)
if rightht == -1:
return -1
if abs(leftht-rightht) > 1:
return -1
return max(leftht,rightht)+1
def isBalanced(self, root):
"""
:type root: TreeNode
:rtype: bool
"""
if self.helper(root) == -1:
return False
return True
```