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/*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.*/

// Recursion

public class Solution {

public List<Integer> grayCode(int n) {

List<Integer> ret = new ArrayList<Integer>();

if (n == 0) {

ret.add(0);

return ret;

}

ret = grayCode(n - 1);

for (int i = ret.size() - 1; i >= 0; i--) {

int num = ret.get(i);

num += 1 << (n - 1);

ret.add(num);

}

return ret;

}

}

// No recursion

public class Solution {

public ArrayList<Integer> grayCode(int n) {

ArrayList<Integer> res=new ArrayList<Integer>();

if(n<0)

return res;

if(n==0)

{

res.add(0);

return res;

}

//n=1的格雷码

res.add(0);

res.add(1);

for(int i=2;i<=n;i++)

{

int num=1<<(i-1);

//第i位格雷码为（第i-1位格雷码前面+0）和（第i-1位格雷码倒序后前面+1）

//而前面+0即为i-1位的格雷码，所以不需要做

//只需要对i-1位进行倒序，然后前面+1

for(int j=res.size()-1;j>=0;j--)

res.add(res.get(j)+num);

}

return res;

}

}

// Discussion