[LeetCode] Gray Code

Problem:

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:O(n)

public class Solution {
    public ArrayList<Integer> grayCode(int n) {
        ArrayList<Integer> result = new ArrayList<Integer>();
 if (n == 0) {
            result.add(0);
            return result;
        }
        result.add(0);
        result.add(1);
        for (int i = 1; i < n; i++) {
            ArrayList<Integer> tmp = new ArrayList<Integer>(result);
     Integer a = 1 << i;
     for (int k = result.size() - 1; k >= 0; k--) {
         tmp.add(result.get(k) + a);
     }
     result = tmp;
 }
 return result;
    }
}

Key Concept:Using reversed order of previous and then set MSB to 1