89. Gray Code M

<span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">问题：</span>

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.

递归即可，假设已有n-1位的graycode，则依次在前面添加上“1”便得到n位的graycode，对应于此题则是加上2^last_graycode_size。注意：是在n-1位graycode中依次从后往前添加“1”，

for (i = gray_len-1 ; i>=0; --i){gray.add(gray.get(i)+add_compare_last_gray);}

Java版：

public class Solution {int add_compare_last_gray = 1;public List<Integer> grayCode(int n) {int i;int gray_len;List<Integer> gray = new ArrayList<Integer>();if (n == 1){gray.add(0);gray.add(1);return gray;}if (n == 0){gray.add(0);return gray;}gray = grayCode(n-1);add_compare_last_gray*=2;gray_len = gray.size();//the gray.size() will change in below loopfor (i = gray_len-1 ; i>=0; --i){gray.add(gray.get(i)+add_compare_last_gray);}return gray;}
}