# Balanced Binary Tree

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

## Example

Given binary tree A = {3,9,20,#,#,15,7}, B = {3,#,20,15,7}

```
A)  3            B)    3 
   / \                  \
  9  20                 20
    /  \                / \
   15   7              15  7
```

The binary tree A is a height-balanced binary tree, but B is not.

## Solution

```
"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None
"""
class Solution:
    """
    @param root: The root of binary tree.
    @return: True if this Binary tree is Balanced, or false.
    """
    def isBalanced(self, root):
        # write your code here
        return self.maxDepth(root) != -1

    def maxDepth(self, root):
        if root == None:
            return 0
        left = self.maxDepth(root.left)
        right = self.maxDepth(root.right)

        if left == -1 or right == -1 or abs(left - right) > 1: # set up -1 is False
            return -1

        return max(left, right) + 1
    #也可以直接使用多参数返回形式。
    def maxDepth(self, root):
        if root == None:
            return True, 0
        left = self.maxDepth(root.left)
        right = self.maxDepth(root.right)
        if left[0] == False or right[0] == False or abs(left[1] - right[1]) > 1:
            return False, 0
        return True, max(left[1], right[1]) + 1
```
