foo.cc: In function 'int main()':
foo.cc:63:51: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::pair<int, int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   63 |                                 for (int p = 0; p < k[a[i][j]].size(); p++)
      |                                                 ~~^~~~~~~~~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务