357. Count Numbers with Unique Digits

题目

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])

Credits:Special thanks to @memoryless for adding this problem and creating all test cases.

解题思路

见程序注释