Count Numbers with Unique Digits

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])
func countNumbersWithUniqueDigits(_ n: Int) -> Int {
        if n == 0 {
            return 1
        }
        var res = 10
        var count = n
        var perCount = 9
        var idx = 9

        while count > 1 && idx > 0 {
            perCount *= idx
            count -= 1
            idx -= 1
            res += perCount
        }
        return res
}