Big Binary Numbers:

Given a number N, find the decimal representation by merging binary equivalent [1..N].

Input N is Long number, Mod output by 10 ^ 9 + 7

Example:
N = 3
1 = 1
2 = 10
3 = 11

Merged binary nums: 11011

Output: 27

Solution[Java]:

 private static long solve( long n ) {
        StringBuilder sb = new StringBuilder();

        for (long i = 1; i <= n; i++) {
            sb.append(Long.toBinaryString(i));
        }

        Long res = new BigInteger(sb.toString(), 2).longValue();
        return (res + (long) (Math.pow(10, 9) + 7)) % (long) (Math.pow(10, 9) + 7);
    }

The above solution is passing only few test cases. Any suggestion, how to write an optimized version?

I am really demotivated after attempting this question. I solved ~400 leetcode problems but stands nowhere in the programming community :(