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。
[CareerCup] 4.1 Balanced Binary Tree 平衡二叉树
原文：http://www.cnblogs.com/grandyang/p/4685660.html