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 - 001 - 111 - 310 - 2Note: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.For now, the judge is able to judge based on one instance of gray code sequence. Sorry about that.

 1 public class Solution { 2     public ArrayList<Integer> grayCode(int n) { 3         ArrayList<Integer> res = new ArrayList<Integer>(); 4         if(n == 0) { 5             res.add(0); 6         } 7         else { 8             ArrayList<Integer> pre = grayCode(n-1); 9             res.addAll(pre);10             for (int i=pre.size()-1; i>=0; i--) {11                 res.add(pre.get(i) + (int)Math.pow(2, n-1));12             }13         }14         return res;15     }16 }