foo.cc: In function 'int main()':
foo.cc:10:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   10 |   for(int j = 0; j < s.size(); j++) a[s[j] - '0']++;
      |                  ~~^~~~~~~~~~
foo.cc:20:10: warning: 'idx' may be used uninitialized in this function [-Wmaybe-uninitialized]
   20 |  cout << idx;
      |          ^~~

[Hydro](https://hydro.ac)提供评测服务