/in/foo.cc: In function 'void era(std::__cxx11::string&, int)':
/in/foo.cc:5:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=x;i<a.length()-1;i++){
               ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:15:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=1;i<st.length()-i;i++) swap(st[i],st[st.length()-i]);
                ^
/in/foo.cc:18:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<st.length()-1-i;i++) swap(st[i],st[st.length()-1-i]);//倒置 
                  ^
/in/foo.cc:13:7: warning: unused variable 'f' [-Wunused-variable]
   int f=0;
       ^
/in/foo.cc:11:16: warning: unused variable 'f' [-Wunused-variable]
  string st;int f=0,f2=0;
                ^
/in/foo.cc:11:20: warning: unused variable 'f2' [-Wunused-variable]
  string st;int f=0,f2=0;
                    ^