枚举二叉树的算法改进

在Python 3中：

def ubtrees(n):    if n == 1:        yield 't'    elif n > 1:        for t in ubtrees(n - 1): yield '(u {})'.format(t)        for i in range(1, n - 1): for t1 in ubtrees(i):     for t2 in ubtrees(n - 1 - i):         yield '(b {} {})'.format(t1, t2)

此递归枚举过程对应于递归

M_1 = 1M_n = M_{n-1} + sum_{i=1}^{n-2} M_i M_{n-1-i},

与Wikipedia中给出的重复率相比偏移了一个。