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.

分析

灰码序列，就是二进制形式的各位只有一个相反。我是用的方法是，从0开始，依次翻转一个位，如果新数字未出现过，就加到结果数组中。然后对该数进一步翻转其中的一个位。该方法循环次数较多，比较慢。
网上有其他方法，找到了该规律： G(i) = i^ (i/2).可以使用。

/**
 * 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=(int*)malloc(sizeof(int)*1000000);
    ans[0]=0;
    *returnSize=1;
    int max=1;
    
    for(int i=0;i<n;i++)
        max=max*2;
    for(int i=1;i<max;i++)
    {
        for(int j=0;j<n;j++)
        {
            int temp=ans[*returnSize-1]^(1<<j);
            int k=0;
            for(k=0;k<*returnSize;k++)
            {
                if(ans[k]==temp)break;
            }
            if(k<*returnSize)
                continue;
            else
            {
                ans[*returnSize]=temp;
                *returnSize=*returnSize+1;
                break;
            }
        }
    }
    return ans;
}