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