Gray Code [Leecode]

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.

Solution: I didn't figure it out until I saw the construction method of the gray code:-( .   Anyway, here has enough info. for you to solve this problem.

   /*******************************SOLUTION 1***********************************/

/*See the definition of gray code: http://en.wikipedia.org/wiki/Gray_code*/
class Solution {
public:
    vector<int> grayCode(int n) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<int> ret;
        if(n<0) return ret;
        ret.push_back(0);
        for(int i=0; i<n; ++i){
            int s = ret.size();
            int highDigit = 1<<i;
            for(int j = s-1; j>=0; --j){
                ret.push_back(ret[j]+highDigit);
            }
        }
        return ret;
    }
};

   /*******************************SOLUTION 2***********************************/

/*See the definition of gray code: http://en.wikipedia.org/wiki/Gray_code*/
class Solution {
public:
    vector<int> grayCode(int n) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<int> ret;
        if(n<0) return ret;
        int size = 1<<n;
        for(int i=0; i<size; ++i)
            ret.push_back(i>>1 ^ i);
        return ret;
    }
};