【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.





观察可以发现，
n位的格雷码前一半元素与n-1的格雷码元素完全相同
后半部分等于前半部分逆序，在加上最高位，即1<<(n-1);

class Solution {
public:
vector<int> grayCode(int n) {

if(n==0) return vector<int>(1,0);

vector<int> preCode=grayCode(n-1);

for(int i=preCode.size()-1;i>=0;i--)
{
int tmp=1<<(n-1);
preCode.push_back(preCode[i]+tmp);
}

return preCode;
}
};