PAT 1049 Counting Ones （30 分）

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.

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Input Specification:
Each input file contains one test case which gives the positive N (≤ $2^{30}$).

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Output Specification:
For each test case, print the number of 1’s in one line.

Sample Input:

12

Sample Output:

5

解析

#include<cstdio>
using namespace std;
int main()
{
	int N, result = 0, left, now, right, product = 1;;
	scanf("%d", &N);
	while (N/product != 0) {
		left = N / (product*10);
		now = N / product % 10;
		right = N % product;
		if (now == 0)	result += left*product; 
		else if (now == 1) result += left * product + right+1;
		else  result += (left + 1)*product;
		product *= 10;
	}
	printf("%d", result);
}