Hackerrank - Sherlock and the Beast

I am trying to solve the Sherlock and The Beast HackerRank challenge. Most tests timeout, however when I try a custom stretch test case (T = 20 and all N = 100000), it returns successfully, so I'm not sure what the problem is.

The idea of the algorithm is that I find the number of possible combinations of threes and fives for a given number of digits, then I treat threes as zeros and fives as ones, in order to process a binary number. So for N = 3 digits, we have 8 combinations (from 7 to 0) which in binary terms is (descending) from 111 to 000, which in three and five terms is from 555 to 333.

public class Solution {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int testCases = in.nextInt();
        for(int i = 0; i =0; i--){          
            String toStr = String.format("%"+n+"s", Long.toBinaryString(i)).replace(' ', '0');

            String modified = toStr.replace("0","3").replace("1","5");
            int threes =0;          
            for( int j=0; j<modified.length(); j++ ) {
                if( modified.charAt(j) == '3' ) {
                    threes++;
                } 
            }                  
            if(threes%5==0) {
                int fives =0;          
                for( int k=0; k<modified.length(); k++ ) {
                    if( modified.charAt(k) == '5' ) {
                        fives++;
                    } 
                }  
                if(fives%3==0) return modified;
            }               
        }
        return "-1";
    }
}

My non-exhaustive list of comments, considering that you seem to prefer (based on observed skill level) plain Java and do not want to use Java 8 yet.

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Adhere to the Java's coding standards. One of the catches here is that method names are in camelCase, so it would be private static String solve(int n) {.

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Intendation and code formatting. You seem to not be using a single standard for all your code. Furthermore I have to say that using spaces where neccessary will not cause any harm, some examples and improvements.

2.1. for(int i = 0; i for (int i = 0; i long combos = (long)Math.pow(2, n);

2.3. if(threes%5==0) { -> if (threes % 5 == 0) {

Please be consistent with your formatting is the bottom line.

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On to the next point, variable names. It does not hurt to use longer variable names if that makes them more meaningful. In your main you have a variable called t, this name is not descriptive, please consider describing what it actually means. Another example is i, ii and iii in your for-loops, it is much more natural to use i, j, k for these variables.

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You could use more abstraction, as your solve method is not easy to follow at all. Consider creating smaller methods that process one step at a time, this will also enable you to be able to test your methods at some point.

StackExchange Code Review Q#47339, answer score: 2