40_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.
1:特殊情况n < 0；2:n分别为0和1的时候；3:分别从后到前遍历数组中的数字，每个数字加上固定的值，添加到数组的后面；固定的值分别为2， 4， 8依次递增

vector<int> grayCode(int n) {
vector<int> result;
if(n < 0)
{
return result;
}

result.push_back(0);

if(n == 0)
{
return result;
}

result.push_back(1);
if(n == 1)
{
return result;
}

int count = 1;
for(int i = 1; i < n; i++)
{
count = count << 1;
for(int j = (int)result.size() - 1; j >= 0; j--)
{
result.push_back(result[j] + count);
}
}
return result;
}