GPT答：

7i24.cn

i, j, sum = 0;     for (i = 0; i < M; i++)         for (j = 0; j < N; j++)             sum += a[i][j];     return sum; }  // 按列先后计算二维数组中的所有元素之和 int sum_array_cols(int a[M][N]) {     int i, j, sum = 0;     for (j = 0; j < N; j++)         for (i = 0; i < M ; i++)             sum += a[i][j];     return sum;}   int main() {      struct timeval start_time,end_time;//定义结构体变量，分别存储开始和结束时间      gettimeofday(&start_time,NULL);//获取当前时间，存入start_time中       int array[M][N];    set_array(array);    printf("Sum of array by rows: %d\n",sum_array_rows(array));    printf("Sum of array by cols: %d\n",sum_array_cols(array));      gettimeofday(&end_time,NULL);//获取当前时间，存入end_time中        long timeuse=1000000*(end_time.tv_sec-start_time.tv_sec)+ end_time.tv_usec-start_time.tv_usec;//计算两个时间点相差的微秒数        printf("Time used:%ldus\n",timeuse);//输出运行所用的总微秒数       return 0;}