pat-a1049. Counting Ones (30)

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这种题也没办法，暂时出现一个学习一个。后面编程问题解决后再来学习数论

这个题时对要求的数的每一位进行分析，分成3部分，left,now,right，now表示现在分析的那个数位上的数，其余两个表示左右的数字，规律程序有，推导不写了。

#include<cstdio>int main(){int n,a=1,ans=0;scanf("%d",&n);while(n/a){int left=n/(a*10);int now=n/a%10;int 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);return 0;}

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.

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:

Sample Output:

0 0