Leetcode 89 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.

Solution 1: simple iterative solution as below

 public class Solution {  
   public List<Integer> grayCode(int n) {  
     List<Integer> res=new ArrayList<>();  
     int size=1;  
     res.add(0);  
     for (int i=1; i<=n; i++) {  
       for (int j=size-1; j>=0; j--) {  
         res.add(1<<i-1|res.get(j));  
       }  
       size*=2;  
     }  
     return res;  
   }  
 }