# Time:  O(n)
# Space: O(h), h is height of binary tree

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


class Solution(object):
    # @param root, a tree node
    # @return a boolean
    def isBalanced(self, root):
        def getHeight(root):
            if root is None:
                return 0
            left_height, right_height = \
                getHeight(root.left), getHeight(root.right)
            if left_height < 0 or right_height < 0 or \
               abs(left_height - right_height) > 1:
                return -1
            return max(left_height, right_height) + 1
        return (getHeight(root) >= 0)