foo.cc: In function 'int find(std::vector<int>&, int)':
foo.cc:7:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for(int i=0;i<a.size();i++)
                ~^~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:26:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.size();i++)
                 ~^~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务