gray c 2b 2b

1	vector <string> a;
2    a={"0","1"};
3    int i, j;
4    for (i = 2; i < (1<<n); i = i<<1)
5    {
6        for (j = i-1 ; j >= 0 ; j--)
7            a.push_back(a[j]);
8        for (j = 0 ; j < i ; j++)
9            a[j] = "0" + a[j];
10        for (j = i ; j < 2*i ; j++)
11            a[j] = "1" + a[j];
12    }
13    for (i = 0 ; i < a.size() ; i++ )
14        cout << a[i] << " ";