/in/foo.cc: In member function 'void FenwickTree::add(int, ll)':
/in/foo.cc:16:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   16 |         while (idx < tree.size()) {
      |                ~~~~^~~~~~~~~~~~~