foo.cc: In function 'int main()':
foo.cc:11:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<ss1.size();i++) {
              ~^~~~~~~~~~~
foo.cc:16:15: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(ch[ss1[i]]==-1) ch[ss1[i]]=ss2[i];
               ^
foo.cc:16:31: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(ch[ss1[i]]==-1) ch[ss1[i]]=ss2[i];
                               ^
foo.cc:17:20: warning: array subscript has type 'char' [-Wchar-subscripts]
   else if(ch[ss1[i]]!=ss2[i]) {
                    ^
foo.cc:28:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<str.length();i++) {
              ~^~~~~~~~~~~~~
foo.cc:29:18: warning: array subscript has type 'char' [-Wchar-subscripts]
   cout<<ch[str[i]];
                  ^