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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0301_0400.s0357_count_numbers_with_unique_digits;

// #Medium #Dynamic_Programming #Math #Backtracking
// #2022_07_11_Time_0_ms_(100.00%)_Space_41.2_MB_(23.67%)

/**
 * 357 - Count Numbers with Unique Digits\.
 *
 * Medium
 *
 * Given an integer `n`, return the count of all numbers with unique digits, `x`, where 0 <= x < 10ⁿ.
 *
 * **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
 *
 * **Constraints:**
 *
 * *   `0 <= n <= 8`
**/
public class Solution {
    public int countNumbersWithUniqueDigits(int n) {
        int ans = 1;
        for (int i = 1; i <= n; i++) {
            int mul = 1;
            for (int j = 1; j < i; j++) {
                mul *= (10 - j);
            }
            ans = ans + 9 * mul;
        }
        return ans;
    }
}