Gray Code

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.

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Submission Result: Wrong AnswerMore Details 

Input:	0
Output:	[]
Expected:

---------------------- codes -----------------------

 class Solution {
 public:
     vector<int> grayCode(int n) {
         vector<int> result;
         //if (n == 0) {
         //    return result;
         //}

         result.push_back(0);
         //result.push_back(1);

         for (int i = 1; i <= n; i++) {
             vector<int> half;
             for (int j = result.size() - 1; j >= 0; j--) {
                 int add = 1<<(i-1);
                 int tmp = result[j];
                 half.push_back(result[j] + (1<<(i-1)));
             }
             result.insert(result.end(), half.begin(), half.end());
         }
         return result;
     }
 };