foo.cc: In function 'int jiao(std::vector<int>)':
foo.cc:30:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   30 |     for (int i = 0; i < po.size() ; i++)
      |                     ~~^~~~~~~~~~~
foo.cc:33:24: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   33 |     for (int i = 0 ; i < nums.size() ; i++)
      |                      ~~^~~~~~~~~~~~~

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