1049. Counting Ones (30)

    《编程之美》有具体分析。

#include <iostream>
#include <cstdio>

using namespace std;

int ones(int n){
	int sum = 0;
	int factor = 1;

	while(n/factor){
		int lower = n - (n/factor)*factor;
		int curr = (n/factor) % 10;
		int higher = n/(factor*10);

		if(curr == 0) sum += higher * factor;
		else if(curr == 1) sum += higher * factor + lower + 1;
		else sum += (higher + 1) * factor;

		factor *= 10;
	}

	return sum;
}

int main(){
	int n;
	scanf("%d", &n);
	printf("%d", ones(n));

	return 0;
}