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.

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

优化精简代码后如下

class Solution {
public:
vector<int> grayCode(int n)
{
vector<int> res;
res.push_back(0);
for (int i = 0; i < n; i++)
{
int high = 1 << i;
for (int j = res.size() - 1; j >= 0; j--)
res.push_back(res[j] + high);
}
return res;
}
};