换顺序

def B(x1):
    if x1 == '':
        return x1
    else:
        return B(x1[1:]) + x1[0]

连乘

def fact(n):
    if n == 1:
        return 1
    else:
        return n * fact(n - 1)

fib

def fib(n):
    if n == 0:
        return 0
    elif n == 1:
        return 1
    else:
        return fib(n-1) + fib(n-2)

def fib(n):
    if n == 0:
        return 0
    elif n == 1:
        return 1
    else:
        return fib(n-1) + fib(n-2)

gold ration

def gold_ratio(n):
    if n == 0:
        return 1
    else:
        return 1 + 1.0 / gold_ratio(n-1)