qsort in c library

qsort shouldn't use a compare function like

int cmp(const void* a, const void* b){
    return *(int*)a - *(int*)b;
}

it would easily cause a overflow, like 2000000000-(-2000000000), it would  overflow and become a negative number.

instead, we should use a compare function like

int cmp(const void* a, const void* b){
    if(*(int*)a > *(int*)b)return 1;
    else if(*(int*)a < *(int*)b)return -1;
    return 0;
}

Swap

Normal

temp=*a; *a=*b; *b=temp;

Advance

*a=*a+*b; *b=*a-*b; *a=*a-*b;

Magic

*a=*a^*b; *b=*a^*b; *a=*a^*b;