4.黑洞数

一般方法：

#!/usr/bin/env python
# encoding: utf-8
"""
@author: 侠之大者kamil
@file: 4.黑洞数.py
@time: 2016/3/21 18:16
"""
# a = [4,6,2,4,5]
# a.sort()
# print(a)
# print(a[::-1])
# def func():
#     pass
# b = 254
# list_b = []
# while b >0:
#     list_b.append(b % 10)
#     b  //= 10
# print(list_b)
# list_b = list_b[::-1]
# s = 0
# for x in list_b:
#     s = s *10 +x
# print(s)
def fun(n):
    print(n)
    a =[]
    k = n
    while k >0:
        a.append(k % 10)
        k //=10
    a.sort()# 小到大排序  [2, 3, 5, 8]
    #print(a)
    s1 = 0
    for x in a[::-1]:#反转
        s1  = s1 *10 +x  # 8532
    s2 = 0
    for x in a:
        s2 = s2 *10 + x # 2358
    if s1 -s2 ==n:
        return n
    else:
        return fun(s1 - s2)
res = fun(1234)
print("rea:",res)

优化后

from functools import reduce
def fun(n):
    print(n)
    a = [int(c) for c in str(n)]
    a.sort()
    s1  = reduce(lambda x,y: 10*x + y ,a[::-1])
    s2  = reduce(lambda x,y: 10*x + y,a)
    return n if s1 -s2 == n else fun(s1 - s2)
res = fun(1234)
print("res:",res)