LeetCode 357.

package lc_357;

public class Solution {
    int[] dp = new int[9];
    public Solution() {
        dp[0] = 1;
        dp[1] = 10;
        dp[2] = 91;
        for(int i=3; i<9; i++) {
            int count = 9;
            int ans = 9;
            for(int j=1; j<i; j++) {
                ans = ans * count;
                count--;
            }
            dp[i] = dp[i - 1] + ans;
        }
    }
    public int countNumbersWithUniqueDigits(int n) {
        return dp[n];
    }
}