用Python怎樣編寫123的反序數？

Python代碼：

def inverse_number(string):

'input a number which is string than,you will get the inverse number'

ans = 0

for i in range(len(string)):

for j in range(i):

if string[j] > string[i]:

ans += 1

return ans

print(inverse_number(input("Please input the number: ")))