p357

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LeetCode 357 Count Numbers with Unique Digits 题解

1.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])

题意：

输入一个数，找出这个数n中所有0 ≤ x < 10n.每个位置上数字都不相同的x的个数

2.解题思路:

排序取中间

3.代码

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public class Solution {
    public int countNumbersWithUniqueDigits(int n) {
       int ans=9;
			int j=9;
			if (n==1)
			{
				return 10;
			}
			if (n==0)
			{
				return 1;
			}
			if (n>10)
			{
				return 8877691;
			}
			for (int i=n;i>1;i--)
			{
				ans=ans*j;
				j--;
			}
			return ans+countNumbersWithUniqueDigits(n-1);
	         
    }
}

4.一些总结：