4.1 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.
LeetCode上的原题，请参见我之前的博客Balanced Binary Tree 平衡二叉树。
本文转自博客园Grandyang的博客，原文链接：平衡二叉树[CareerCup] 4.1 Balanced Binary Tree ，如需转载请自行联系原博主。
