【1049】Counting Ones (30 分)

#include<iostream>
#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<string.h>
#include<algorithm>  
#include<map>
#include<vector>
#include<queue> 
using namespace std;  
//找到1~n的all数里含1的1的个数,对逐位分三种情况
//代码虽短，但要举例静心找规律
   
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);
    system("pause");
    return 0;   
}