Problem:
Given a non-negative integer n, count all numbers with unique digits, x, where 0 ≤ x < 10n.
Example:
Input: 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
思路：
Solution (C++):
int countNumbersWithUniqueDigits(int n) {
int unique_count = 0;
for (int i = 0; i <= n; ++i) {
unique_count += no_duplicate(i);
}
return unique_count;
}
int no_duplicate(int n) {
if (n == 0) return 1;
int count = 9;
for (int i = 1; i < n; ++i) {
count *= (10-i);
}
return count;
}
性能：
Runtime: 0 ms Memory Usage: 6.1 MB
思路：
Solution (C++):
性能：
Runtime: ms Memory Usage: MB
