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.

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 }