foo.cc: In function 'int main()':
foo.cc:56:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<node>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   56 |     for(int i=0;i<list.size();i++){  //循环每一个商品
      |                 ~^~~~~~~~~~~~

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