题目：

题解：

int* grayCode(int n, int* returnSize) {
    int ret_size = 1 << n;
    int *ret = (int *)malloc(ret_size * sizeof(int));
    for (int i = 0; i < ret_size; i++) {
        ret[i] = (i >> 1) ^ i;
    }
    *returnSize = ret_size;
    return ret;
}