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.