foo.cpp: In function 'void calcnext(std::vector<int>&, std::string)':
foo.cpp:10:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while (j<s.length()){
                    ^
foo.cpp: In function 'std::string make_string()':
foo.cpp:37:24: error: 'to_string' was not declared in this scope
   result += to_string(i);
                        ^