357. Count Numbers with Unique Digits
Math
Dynamic Programming
Backtracking
Given an integer n, return the count of all numbers with unique digits, x, where 0 <= x < 10n.
Examples
Example 1
Input: n = 2
Output: 91
Explanation: The answer should be the total numbers in the range of 0 ≤ x < 100, excluding 11,22,33,44,55,66,77,88,99
Example 2
Input: n = 0
Output: 1