[CareerCup] 4.1 Balanced Binary Tree 平衡二叉树

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。