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 - 001 - 111 - 310 - 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.

这道题可以用迭代的方法在数字之前加1，每次从list的最后一位向前迭代。代码如下：

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