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])

代码如下：

 1 public class Solution {
 2     public int countNumbersWithUniqueDigits(int n) {
 3         if(n==0)
 4         return 1;
 5         int sum=0,p=0;
 6         
 7         if(n<=2)
 8         return (int)Math.pow(9,n)+countNumbersWithUniqueDigits(n-1);
 9         else{
10             sum=countNumbersWithUniqueDigits(n-1);
11             n=n-2;
12             p=81;
13             int q=8;
14             while(n>0)
15             {
16                 p=p*q;
17                 q--;
18                 n--;
19             }
20         
21         }
22         return sum+p;
23     }
24 }