Gray Code.java

/*
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
Note:
For a given n, a gray code sequence is not uniquely defined.

For example, [0,2,3,1] is also a valid gray code sequence according to the above definition.

For now, the judge is able to judge based on one instance of gray code sequence. Sorry about that.
*/

public class Solution {
    public ArrayList<Integer> grayCode(int n) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<Integer> result = new ArrayList<Integer>();
        result.add(0);
        
        if (n == 0) return result;
        
        result.add(1);
        
        for (int i = 1; i < n; i++) {
            ArrayList<Integer> temp = result;
            int m = 1 << i;
            for (int j = result.size() - 1; j >= 0; j--) {
                temp.add(m + result.get(j));
            }
            result = temp;
        }
        
        return result;
    }
}