Gray Code

题目：

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.

思路：最开始用的方法是递归，例如，n=3可以看成是n=2的结果尾巴添加n=2的结果加上2的2次方的倒序。

class Solution {
public:
    vector<int> grayCode(int n) {
        vector<int> result;
        if(n==0){
            result.push_back(0);
            return result;
        }
        if(n==1){
            result.push_back(0);
            result.push_back(1);
            return result;
        }
        result=grayCode(n-1);
        int size=result.size();
        int i=size-1;
        int temp;
        for(;i>=0;--i){
            temp=size+result[i];
            result.push_back(temp);
        }
        return result;
            
    }
    
};

第二种方便的思路是将二进制码右移一位与本身异或即可得到结果