LeetCode: Gray Code

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


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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<Integer> prev = new ArrayList<Integer>();
        for(int i=0;i<n;i++){
            ArrayList<Integer> cur = new ArrayList<Integer>();
            for(Integer temp:prev){
                cur.add(temp);
            }
            cur.add(i+1);
            for(Integer temp:prev){
                cur.add(temp);
            }
            prev=new ArrayList<Integer>(cur);
        }
        
        int start =0;
        ArrayList<Integer> res = new ArrayList<Integer>();
        res.add(start);
        
        for(Integer temp:prev){
            start=flipAtPosition(start,temp);
            res.add(start);
        }
        return res;
    }
    
    public int flipAtPosition(int a,int k){
        int temp = (a>>(k-1))&1;
        if(temp==1) return a-(1<<(k-1));
        else return a+(1<<(k-1));
    }
}