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.

解法：使用递归或者迭代的思想。每次增加一位，最高位就增加了0，后者1.最高位是0的情况就是n-1的解，最高位是1的情况就是n-1的解的倒序加上一个2的指数。

public class Solution {
    List<Integer> before=new ArrayList<>(Arrays.asList(0,1));
    int pluss=1;
    public List<Integer> grayCode(int n) {
            List<Integer> list=new ArrayList<>();
            list.add(0);
            if (n==0) {
                return list;
            }
            if (n==1) {
                list.add(1);
                return list;
            }
            list.add(1);
            for (int i = 1; i < n; i++) {
                pluss=pluss*2;
                for (int j = before.size()-1; j >=0 ; j--) {
                    list.add(before.get(j)+pluss);
                }
                before=list;
            }
            return list;
    }
}