foo.cc: In function 'int output(std::vector<Friend>)':
foo.cc:11:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<k&&i<a.size();i++)
                   ~^~~~~~~~~
foo.cc:13:1: warning: no return statement in function returning non-void [-Wreturn-type]
 }
 ^
foo.cc: In function 'bool cmp(const Friend&, const Friend&)':
foo.cc:20:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^

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