格雷码的定义

第二次写的，参考别人。代码写的很简洁

 1 class Solution {
 2 public:
 3     vector<int> grayCode(int n) {
 4         // Start typing your C/C++ solution below
 5         // DO NOT write int main() function
 6         vector<int> res;
 7         res.push_back(0);
 8         int highbit = 0;
 9         while(n--)
10         {
11             highbit = res.size();
12             for(int i= res.size()-1; i >= 0; i--)
13                 res.push_back(highbit|res[i]);
14         }
15         return res;
16     }
17 };