## Question

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.

## Solution

**Result:** Accepted **Time:** 0 ms

Here should be some explanations.

```
/**
* Return an array of size *returnSize.
* Note: The returned array must be malloced, assume caller calls free().
*/
int* grayCode(int n, int* returnSize) {
int * ans = malloc(sizeof(int) * (1<<n));
*returnSize = 1 << n;
for(int i = 0; i < (1<<n); i++)
ans[i] = 0;
for(int i = 0; i < n; i++)
{
int t = 0;
int bit = 1 << i;
for(int j = 0 ; j < (1<<n); j+=bit)
{
if(t == 1 || t == 2)
for(int k = 0; k < bit; k++)
ans[j + k] |= bit;
t++;
t &= 3;
}
}
return ans;
}
```

**Complexity Analytics**

- Time Complexity:
- Space Complexity: