PAT甲级1049. Counting Ones (30)

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

//看了别人的才知道。很巧妙的方法 
#include 
using namespace std;

int main(){
    int N;
    cin>>N;

    int count=0;
    int x=1;
    while(N/x>0){
        int current=(N/x)%10;
        int left=N/(10*x);
        int right=N%x;
        if(current<1) count+=left*x;
        else if(current==1) count+=(left*x+right+1);
        else count+=(left+1)*x;
        x*=10;
    }
    cout<return 0;
}