/in/foo.cc: In function 'bool isJolly(const std::vector<int>&)':
/in/foo.cc:17:25: warning: comparison of integer expressions of different signedness: 'std::unordered_set<int>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   17 |     return diffs.size() == n - 1; // 是否覆盖1到n-1
      |            ~~~~~~~~~~~~~^~~~~~~~