foo.cc: In function 'void solve(std::__cxx11::string)':
foo.cc:27:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.length();i++){
              ~^~~~~~~~~~~
foo.cc:39:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=1;i<ans.length();i++){
              ~^~~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:55:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.length();i++){
              ~^~~~~~~~~~~

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