/in/foo.cc: In function 'int main()':
/in/foo.cc:9:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < a.size(); i++) {
                  ~~^~~~~~~~~~
/in/foo.cc:11:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   while (j < a.size()) {
          ~~^~~~~~~~~~
/in/foo.cc:12:8: warning: unused variable 'flag' [-Wunused-variable]
    int flag = false;
        ^~~~