foo.cc: In function 'int main()':
foo.cc:50:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   50 |  for (int i = 0; i < s.length(); i++)
      |                  ~~^~~~~~~~~~~~
foo.cc: In function 'ElemType Pop(SeqStack&)':
foo.cc:25:1: warning: control reaches end of non-void function [-Wreturn-type]
   25 | }
      | ^

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