[Leetcode] 89. 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 - 001 - 111 - 310 - 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.

import java.util.ArrayList;public class Solution {    public ArrayList<Integer> grayCode(int n) {        ArrayList<Integer> result = new ArrayList<Integer>();        if(n < 0) return result;        if(n == 0){            result.add(0);            return result;        }        ArrayList<Integer> last = grayCode(n - 1);        int addNum = 1 << (n - 1);        for(int i = last.size() - 1; i >= 0; i--){            last.add(last.get(i) + addNum);        }        return last;    }}