很久没写C语言的代码，发现很多小细节。

0.C语言常规头文件

#include <stdlib.h>
#include <stdio.h>

1. 二维数组的开辟和释放-malloc()&free()

double ** a;   //a[m][n]
a = (double **) malloc (sizeof(double *) *m);
    for (int i = 0; i < m; i ++)
        a[i] = (double *) malloc (sizeof(double) *n);

for (int i = 0; i < m; i ++){
    free(a[i]);
}
free(a);