leetcode - Gray Code

class Solution {
public:
vector<int> grayCode(int n) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
vector<int> rlt;
rlt.push_back(0);
if (n<=0)
return rlt;
int i;
for (i=0; i<n; i++){
int addNum = 1<<i;
vector<int>::iterator ii = rlt.end()-1;
vector<int> tmpVec;
int size = rlt.size();
while (size-- > 0){
tmpVec.push_back(*ii + addNum);
ii--;
}
size = rlt.size();
ii = tmpVec.begin();
while(size-- >0 )
rlt.push_back(*(ii++));
}
return rlt;
}
};