爱奇艺已知错位和SUM求原数X的题目的完美解法

#只用数字a组成order位数
#例如extend(2,3)=222, extend(4,2)=44
def extend(a, order):
     result = 0
     while order>0:
         order -= 1
         result = result*10 + a
     return result

#返回数字n的最高位数字，以及这是第几位。
#个位按第一位算起
def topDigit(n):
     order = 1
     while n >= 10:
         n //= 10
         order += 1
     return n, order

def initn(s):
     n = int(s)
     cur = None
     last_order = len(s) + 1
     result = 0
     while n != 0:
         n0, order = topDigit(n)
         extend_n0 = extend(n0, order)
         if n >= extend_n0:
             cur = n0
             n -= extend_n0
         elif n0 == 1:
             cur = 9
             order -= 1
             n -= extend(9, order)
         else:
             cur = n0 -1
             n -= extend(cur, order)
         if last_order == order:
             return -1
         result += cur * 10**(order-1)
         last_order = order
     return result