/in/foo.cc: In constructor 'SeqList::SeqList(int)':
/in/foo.cc:14:11: warning: converting to non-pointer type 'int' from NULL [-Wconversion-null]
    a[i] = NULL;
           ^~~~
/in/foo.cc: In member function 'SeqList SeqList::Jiao(SeqList&)':
/in/foo.cc:32:16: warning: converting to non-pointer type 'int' from NULL [-Wconversion-null]
    temp.a[k] = NULL;
                ^~~~
/in/foo.cc: In member function 'void SeqList::Output()':
/in/foo.cc:52:15: warning: NULL used in arithmetic [-Wpointer-arith]
    if(a[i] != NULL)
               ^~~~