A1049 Counting Ones

1049 Counting Ones （30 分)

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 (≤).

Output Specification:

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

Sample Input:

12

Sample Output:

5

#include 
#include 
#include 
using namespace std;

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", ans);
	return 0;
}