foo.cc: In function 'P decode(const V&)':
foo.cc:37:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   37 |   for(int i=1;i+1<s.size();i+=2){
      |               ~~~^~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务