Count Numbers with Unique Digits
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1问题描述
Given a non-negative integer n, count all numbers with unique digits, x, where 0 ≤ x < 10的n次方.
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])
2.思路.
利用排列组合来解决.
3代码
class Solution {public: int permutation(int n, int r) { if(r == 0) { return 1; }else{ return n * permutation(n - 1, r - 1); } } int countNumbersWithUniqueDigits(int n) { int sum = 1; if(n > 0) { int end = (n > 10)? 10 : n; for(int i = 0; i < end; i++) { sum += 9 * permutation(9, i); } } return sum; }};
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