[C++知识库]1049Counting Ones

The task is simple: given any positive integer?N, you are supposed to count the total number of 1's in the decimal form of the integers from 1 to?N. For example, given?N?being 12, there are five 1's in 1, 10, 11, and 12.

Input Specification:

Each input file contains one test case which gives the positive?N?(≤230).

Output Specification:

For each test case, print the number of 1's in one line.

Sample Input:

12

Sample Output:

5

编译软件：visual studio

编译语言：c语言

参考代码：

#include<cstdio>

int main()
{
?? ?int n, a = 1, ans = 0;
?? ?int left, now, right;
?? ?scanf("%d", &n);
?? ?while (n / a != 0)
?? ?{
?? ??? ?left = n / (a * 10);
?? ??? ?now = n / a % 10;
?? ??? ?right = n % a;
?? ??? ?if (now == 0)
?? ??? ??? ?ans += left * a;
?? ??? ?else if (now == 1)ans += left * a + right + 1;
?? ??? ?else ans += (left + 1) * a;
?? ??? ?a *= 10;
?? ?}
?? ?printf("%d\n", ans);
?? ?return 0;
}