foo.cc: In function 'bool check()':
foo.cc:35:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (int k = 0; k < s.size(); k++)
                 ~~^~~~~~~~~~
foo.cc:39:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (int k = 0; k < s.size(); k++)
                 ~~^~~~~~~~~~

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