二叉树也是递归定义的，其结点有左右子树之分，逻辑上二叉树有五种基本形态： (1)空二叉树——(a)； (2)只有一个根结点的二叉树——(b)； (3)右子树为空的二叉树——(c)； (4)左子树为空的二叉树——(d)； (5)完全二叉树——(e)注意：尽管二叉树与树有许多相似之处，但二叉树不是树的特殊情形。

-Binary tree is recursively defined, and its sub-tree nodes are so divided, there are five basic logical form of binary tree: (1) empty binary tree- (a)　(2) there is only one root node Binary Tree- (b)　(3) the right subtree of the binary tree is empty- (c)　(4) the left subtree of the binary tree is empty- (d)　(5) completely Binary Tree- (e) Note: Although the binary tree and the tree has many similarities, but the binary tree is not a special case.