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

参考了别人的思路：http://bangbingsyb.blogspot.com/2014/11/leetcode-gray-code.html

思路：有规律：gray(n)=gray(n-1)+(gray(n-1)+2^(n-1))。直接递归做即可

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

89. Gray Code