Leetcode: Gray Code

The gray code is a binary numeral system where two successive values differ in only one bit.

Given a non-negative integer n representing the total number of bits in the code, print the sequence of gray code. A gray code sequence must begin with 0.

For example, given n = 2, return [0,1,3,2]. Its gray code sequence is:

00 - 0
01 - 1
11 - 3
10 - 2

Note:
For a given n, a gray code sequence is not uniquely defined.

For example, [0,2,3,1] is also a valid gray code sequence according to the above definition.

Solution:

public class Solution {
    public ArrayList<Integer> grayCode(int n) {
        ArrayList<Integer> solution = new ArrayList<Integer>();
        grayCode(0, n, new boolean[(int)Math.pow(2,n)],solution);
        return solution;
    }
    
    
    
    
    public boolean grayCode(int number, int n, boolean[] used, ArrayList<Integer> solution)
    {
        used[number] = true;
        solution.add(number);
        if (solution.size()== (int) Math.pow(2,n))
            return true;
        for(int i=0; i<n; i++)
        {
           int number2 = number ^ (1<<i);
           if(!used[number2])
           {
                if(grayCode(number2, n, used, solution))
                    return true;
           }
        }
        solution.remove(solution.size()-1);
        used[number]=false;
        return false;
    }
}