#### Recurrence relation listed in `docstring`

of method

```
class Solution(object):
def countNumbersWithUniqueDigits(self, n):
'''
n = 0 -> 1
n = 1 -> 9 + countNumbersWithUniqueDigits(0) -> 10
n = 2 -> 9 * 9 * 8 + countNumbersWithUniqueDigits(1) -> 739
n = 3 -> 9 * 9 * 8 * 7 + countNumbersWithUniqueDigits(1) -> 5275
'''
if n == 0:
return 1
else:
uniques = 9
if range(9,10-n,-1):
uniques *= reduce(lambda x,y:x*y, range(9,10-n,-1))
return uniques + self.countNumbersWithUniqueDigits(n-1)
```