foo.cc: In function 'void unique(node*)':
foo.cc:66:29: warning: variable 'pre2' set but not used [-Wunused-but-set-variable]
   66 |         node* pre = NULL, * pre2 = NULL,*i,*j;
      |                             ^~~~
foo.cc: In function 'int main()':
foo.cc:104:24: warning: unused variable 'i' [-Wunused-variable]
  104 |         int n1, n2, n3,i, t;
      |                        ^
foo.cc:104:27: warning: unused variable 't' [-Wunused-variable]
  104 |         int n1, n2, n3,i, t;
      |                           ^

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