https://www.hackerrank.com/contests/hackerglau/challenges/lostpages/

LostPagec.cpp

//Here is my code implemented in c++
#include <bits/stdc++.h>
#define gc getchar_unlocked()
#define pf printf 
using namespace std;
long pages[1000001], prfx[2000001];
 
int fr() //fast input
{
    int n = 0;
    char c = gc;
    while (!('0' <= c && c <= '9')) {
        c = gc;
    }
    while ('0' <= c && c <= '9') {
        n = n * 10 + c - '0';
        c = gc;
    }
    return n;
}
long f(int i)
{
    int odd = 0, even =0;
    while(i > 0){
        int temp = i% 10;
        if(temp % 2 == 0)
        {
            even += temp;
        }
        else odd += temp;            
        i = i/10;
    }
    return abs(odd - even);
}
int main()
{
    //ios_base::sync_with_stdio(false);
    //cin.tie(NULL);
    
   
    for(int i = 1; i <2000001; i++)
    {
        prfx[i] = prfx[i-1]+f(i);
    }
    for(int i = 1; i < 1000001; i++)
    {
        pages[i] = pages[i-1] + 2 * (prfx[2 * i] - prfx[i]) - f(2 *i);
    }
    long n, t;
    //t = fr(); 
    scanf("%lld",&t);
    while(t--)
    {
        //n = fr();
        scanf("%lld",&n);
        pf("%lld\n", pages[n]);
    }   
    return 0;
}