foo.cc: In function 'void unique(node*)':
foo.cc:71:29: warning: variable 'pre2' set but not used [-Wunused-but-set-variable]
   71 |         node* pre = NULL, * pre2 = NULL, * i, * j;
      |                             ^~~~
foo.cc: In function 'int FindMaxNeg(node*)':
foo.cc:152:1: warning: no return statement in function returning non-void [-Wreturn-type]
  152 | }
      | ^
foo.cc: In function 'int main()':
foo.cc:155:21: warning: unused variable 'n3' [-Wunused-variable]
  155 |         int n1, n2, n3, i, t;
      |                     ^~
foo.cc:155:25: warning: unused variable 'i' [-Wunused-variable]
  155 |         int n1, n2, n3, i, t;
      |                         ^
foo.cc:155:28: warning: unused variable 't' [-Wunused-variable]
  155 |         int n1, n2, n3, i, t;
      |                            ^
foo.cc:156:15: warning: unused variable 'head4' [-Wunused-variable]
  156 |         node* head4 = new node, * head5 = new node;
      |               ^~~~~
foo.cc:156:35: warning: unused variable 'head5' [-Wunused-variable]
  156 |         node* head4 = new node, * head5 = new node;
      |                                   ^~~~~
foo.cc:158:31: warning: unused variable 'head3' [-Wunused-variable]
  158 |         node* head1 = NULL, * head3 = NULL, * head2 = NULL;
      |                               ^~~~~

[Hydro](https://hydro.ac)提供评测服务