A Full Binary Tree (FBT) is a tree in which every node other than the leaves has two children.

换句话说，所有非叶子结点的度都是2。（只要你有孩子，你就必然是有两个孩子。）

**注：**Full Binary Tree又叫做Strictly Binary Tree。

例如：

有N个节点的满二叉树的高度