Implement a function to check if a binary tree is balanced. For the purposes of this question, a balanced tree is defined to be a tree such that the heights of the two subtrees of any node never differ by more than one.

Example 1:

Given tree [3,9,20,null,null,15,7]
    3
   / \
  9  20
    /  \
   15   7
return true.

Example 2:

Given [1,2,2,3,3,null,null,4,4]
      1
     / \
    2   2
   / \
  3   3
 / \
4   4
return false.