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

There might be multiple gray code sequences possible for a given n. Return any such sequence.

// Let G(n) represent a gray code of n bits.
// Note that reverse of G(n) is also a valid sequence.
// Let R(n) denote the reverse of G(n).

// Note that we can construct
// G(n+1) as the following :
// 0G(n)
// 1R(n)

// Where 0G(n) means all elements of G(n) prefixed with 0 bit and 1R(n) means all elements of R(n) prefixed with 1.
// Note that last element of G(n) and first element of R(n) is same. So the above sequence is valid.

// Example G(2) to G(3) :
// 0 00
// 0 01
// 0 11
// 0 10
// 1 10
// 1 11
// 1 01
// 1 00
public class Solution {
    public List<Integer> grayCode(int a) {
        int len = 1 << a;
        List<Integer> res = new ArrayList<>(len);
        res.add(0);
        for (int i = 1; i <= a; i++) {
            for (int j = res.size() - 1; j >= 0; j--) {
                int next = res.get(j) | (1 << (i - 1));
                res.add(next);
            }
        }
        return res;
    }
}