Gray Code @LeetCode

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.

	package leetcode;
	import java.util.ArrayList;
	/**
	* Solution: GrayCode(n=k), is equals to GrayCode(n = k-1) reverse + 1<<k. Calculate Layer by Layer.
	* The gray code -- two successive values differ in only one bit. But there's many combinations for a n > 2.
	*
	* 0 00 0
	* 0 01 1
	* 0 11 3
	* 0 10 2
	* ----
	* 1 10 2 + 4
	* 1 11 3 + 4
	* 1 01 1 + 4
	* 1 00 0 + 4
	*
	* Reference: http://fisherlei.blogspot.com/2012/12/leetcode-gray-code.html
	*
	* @author jeffwan
	* @date Apr 4, 2014
	*/
	public class GrayCode {
	public ArrayList<Integer> grayCode(int n) {
	ArrayList<Integer> result = new ArrayList<Integer>();
	if (n < 0) {
	return result;
	}
	result.add(0);
	for (int i = 0; i < n; i++) {
	int highestBit = 1 << i;
	int length = result.size();
	for (int j = length - 1; j >= 0; j--) {
	result.add(highestBit + result.get(j));
	}
	}
	return result;
	}
	// Mathematical Solution: i --> (i>>1)^i No meaning in interview.
	public ArrayList<Integer> grayCode2(int n) {
	ArrayList<Integer> result = new ArrayList<Integer>();
	if (n < 0) {
	return result;
	}
	int size = 1 << n;
	for (int i = 0; i < size; i++) {
	result.add((i >> 1) ^ i);
	}
	return result;
	}
	}

view raw GrayCode.java hosted with ❤ by GitHub