foo.cc: In function ‘int main()’:
foo.cc:10:19: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   10 |     for(auto i=0;i<str.length();i++){
      |                  ~^~~~~~~~~~~~~
foo.cc:15:22: warning: comparison of integer expressions of different signedness: ‘std::stack<int>::size_type’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]
   15 |         if(sta.size()>ans)
      |            ~~~~~~~~~~^~~~