【Backup】negative array index

It means the same as a positive index: a[b] is equivalent to *(a + b),
so a[-1] is equivalent to *(a - 1)

If you have a pointer that points into an array (i.e. not at the first
element), a negative index is perfectly legal

eg.

int a[5] = {1, 2, 3, 4, 5};

int *b = a+3；

则 b[-1] = 3;

   b[-2] = 2;