HackerRank "Digit sum" challenge

Question

Here's the question:enter image description here

 Scanner scanner = new Scanner(System.in); 
 long x = scanner.nextLong();
 
 
 if(x<=1) return;
 if(x>=Math.pow(10, 12)) return;
 
 Map<Integer, Long> y = new HashMap<Integer, Long>();
 long k = 5;
 long yes = 0;
 
 for(int f = 0;f<x; f++){
 y.put(f, k);
 k++;
 
 
 if(k==7){
 k=1;
 }
 yes+= y.get(f);
 if(f == x-1){
 System.out.println((long)yes);
 }
 
 } 
// for (int g = 0; g<y.size(); g++) {
// yes+= y.get(g);
// } System.out.println((long)yes);
 
 
 
 
 
}}

The first time I was using ArrayList instead of HashMap. I changed it yet it keeps giving me the error. I even tried combining the loop as you can see from the commented out code.

The code works for the first few test cases but after it got into the 7+ digits (~2500000) it starts timing me out.

Please go easy on me as I just started learning Java a few days ago but I have decent JavaScript knowledge.

Do provide me tips on how to increase code efficiency in the future if I ever run into this again.

• Toby Speight: Some of the code (e.g. imports) seems to be missing. It's hard to review code that is incomplete.

Answer 1

Your algorithm, which works one digit at a time, can be switched to one that's much more efficient.

See what happens if you divide the long string of digits into blocks of 6:

561234 561234 561234 561234 561234 561234 ...

Each of those blocks has the same sum, so we can add them all up quite easily:

sum = x / 6 * 21;

For the partial block remaining, we could use a simple look-up table:

static final int[] partial = new int[6]{ 0, 5, 11, 12, 14, 17 };
sum += partial[x % 6];

This solution scales much better, as we no longer need a loop.

• The Infinite Star: It works! but it gives an input mismatch exception after 10 digits. What is causing it to do so?
• The Infinite Star: Oh gosh my bad. I used nextInt() instead of nextLong(). Sorry! it works nicely now.