/in/foo.cc: In function 'int main()':
/in/foo.cc:8:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.length()-2;i++)
              ~^~~~~~~~~~~~~
/in/foo.cc:12:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for(int j=i+1;j<a.length()-1;j++)
                  ~^~~~~~~~~~~~~
/in/foo.cc:16:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
      for(int k=j+1;k<a.length();k++)
                    ~^~~~~~~~~~~