foo.cc: In function 'int main()':
foo.cc:10:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < a.length(); i++)
                  ~~^~~~~~~~~~~~
foo.cc:12:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int j = i + 1; j < a.length() + 1; j++)
                       ~~^~~~~~~~~~~~~~~~
foo.cc:15:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (int k = 0; k < b.length(); k++)
                    ~~^~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务