Given a non-negative integer n, count all numbers with unique digits, x, where 0 ≤ x < 10n.
Example: Given n = 2, return 91. (The answer should be the total numbers in the range of 0 ≤ x < 100, excluding [11,22,33,44,55,66,77,88,99])
Given a non-negative integer n, count all numbers with unique digits, x, where 0 ≤ x < 10n.
Example: Given n = 2, return 91. (The answer should be the total numbers in the range of 0 ≤ x < 100, excluding [11,22,33,44,55,66,77,88,99])