foo.cc: In function 'int main()':
foo.cc:21:22: warning: NULL used in arithmetic [-Wpointer-arith]
     while (p->num != NULL)
                      ^~~~
foo.cc:12:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d%d", &n, &m);
     ~~~~~^~~~~~~~~~~~~~~~
foo.cc:14:14: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         scanf("%d%d", &vn[i].id, &vn[i].num);
         ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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