1049. Counting Ones (30)

题目：https://www.patest.cn/contests/pat-a-practise/1049

#include<cstdio>int main(){  int n,a=1,left,now,right,ans=0;  scanf("%d",&n);  while(n/a)  {    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;}