Gray Code

    //reference http://blog.csdn.net/fightforyourdream/article/details/14517973
    vector<int> grayCode(int n) {
        vector<int> codes;
        if(n<0)
            return codes;
        int size=1<<n;//相当于2的n次方
        int i,tmp;
        for(i=0;i<size;++i)
        {
            tmp=i^i>>1;
            codes.push_back(tmp);
        }
        return codes;
    }