设二叉树中每个结点的元素均为一个字符，按先序遍历的顺序建立二叉链表，编写三个递归算法对二叉树的结点（度为0、1、2）个数进行统计。

递归算法如下：

class Node:
    def __init__(self, val=None, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def count_leaf(node):
    if not node:
        return 0
    if not node.left and not node.right:
        return 1
    return count_leaf(node.left) + count_leaf(node.right)

def count_degree_one(node):
    if not node:
        return 0
    if (node.left and not node.right) or (node.right and not node.left):
        return 1
    return count_degree_one(node.left) + count_degree_one(node.right)

def count_degree_two(node):
    if not node:
        return 0
    if node.left and node.right:
        return 1
    return count_degree_two(node.left) + count_degree_two(node.right)

其中 `count_leaf` 统计叶子结点个数， `count_degree_one` 统计度为1的结点个数， `count_degree_two` 统计度为2的结点个数。