[leetcode] 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.

思路：看看n=3时的情况就知道了，000，001，011，010，110，111，101，100，就是在n=2的情况下，前面加0和逆序加1

代码：

class Solution {
public:
vector<int> grayCode(int n) {
vector<int> res;
if(n==0){
res.push_back(0);
return res;
}
vector<int> temp=grayCode(n-1);
res=temp;
int len=temp.size();
for(int i=len-1;i>=0;i--){
res.push_back(temp[i]+pow(2,n-1));
}
return res;
}
};