foo.cc: In function 'void decode(std::string&)':
foo.cc:5:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    5 |  for(int i=0;i<a.length();i++){
      |              ~^~~~~~~~~~~

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