二叉查换树，左孩子小于等于根，右孩子大于根。
完全二叉树，叶子都在最后一层，所有结点（除了叶子）都有两个孩子。
平衡二叉树，左右子树的高度在一定范围内。
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.
点此。
4.2
Careercup | Chapter 4,布布扣,bubuko.com
原文地址：http://www.cnblogs.com/linyx/p/3782607.html
