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

就是找规律，把上一层得到的结果逆序前面一位置1，然后把这些新数加回结果中

public class Solution {
    //Time: O(n^2)  Space: O(n^2)
    public ArrayList<Integer> grayCode(int n) {
        ArrayList<Integer> res = new ArrayList<Integer>();
        res.add(0);
        if (n <= 0) {
            return res;
        }
        
        res.add(1);
        for (int i = 1; i < n; i++) {
            for (int j = res.size() - 1; j >= 0; j--) {
                int cur = res.get(j);
                cur |= (1 << i);
                res.add(cur);
            }
        }
        return res;
    }
}