foo.cc: In function 'int main()':
foo.cc:8:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s.size() - 1; i++) {
                  ~~^~~~~~~~~~~~~~
foo.cc:18:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < s.size() - 1; i++) {
                   ~~^~~~~~~~~~~~~~

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