/in/foo.cc: In function 'int main()':
/in/foo.cc:31:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < (s[2].length() + 1) / 2; i++)  //还原1
                  ~~^~~~~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc:60:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < (s[1].length() + 1) / 2; i++)  //还原1
                  ~~^~~~~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc:89:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < (s[0].length() + 1) / 2; i++)  //还原1
                  ~~^~~~~~~~~~~~~~~~~~~~~~~~~