POJ 1019 Number Sequence

 

#include <iostream>
#include <cstdio>
#include <cmath>

using namespace std;

int main()
{
    int t,n,i,digit_now,remain_digit,num,pos,ans;
    long long int sum_digit;
    scanf("%d",&t);
    while (t--)
    {
        scanf("%d",&n);
        for (i=1,sum_digit=0,digit_now=0;sum_digit<n;i++)
        {
            digit_now+=(int)log10((double)i)+1;
            sum_digit+=digit_now;
        }
        sum_digit-=digit_now;
        remain_digit=n-sum_digit;
        for (i=1,digit_now=0;digit_now<remain_digit;i++)
            digit_now+=(int)log10((double)i)+1;
        num=i-1;
        digit_now-=(int)log10((double)num)+1;
        pos=(int)log10((double)num)+2-(remain_digit-digit_now);
        for (i=1;i<pos;i++)
            num/=10;
        ans=num%10;
        printf("%d\n",ans);
    }
    return 0;
}