foo.cc: In function 'int find(std::vector<int>, int)':
foo.cc:5:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    5 |  for (int i = 0; i < b.size(); i++) {
      |                  ~~^~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:32:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   32 |  for (int i = 0; i < a.size(); i++) { //差集的特点
      |                  ~~^~~~~~~~~~
foo.cc:37:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   37 |  for (int i = 0; i < c.size(); i++) {
      |                  ~~^~~~~~~~~~
foo.cc:19:6: warning: unused variable 'NA' [-Wunused-variable]
   19 |  int NA = na;
      |      ^~
foo.cc:26:6: warning: unused variable 'NB' [-Wunused-variable]
   26 |  int NB = nb;
      |      ^~

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