foo.cc: In function 'void dfs(int, int)':
foo.cc:41:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if (now==f.size()){
      ~~~^~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:59:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0; i<f.size(); i++) code[i]=f[i];
                ~^~~~~~~~~
foo.cc:64:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int j=0; j<s.size(); j++){
                 ~^~~~~~~~~

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