foo.cc: In function 'void solve()':
foo.cc:47:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.length();i++){
              ~^~~~~~~~~~~
foo.cc:53:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=1;i<s.length();i++){
              ~^~~~~~~~~~~
foo.cc: In function 'void BTS(std::__cxx11::string&)':
foo.cc:59:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.length();i++){
              ~^~~~~~~~~~~

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