Compilare: user.cpp: In function ‘int main()’: user.cpp:24:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=1;i<words.size();i++) ^ user.cpp:26:32: warning: array subscript has type ‘char’ [-Wchar-subscripts] prev[i] = ch[words[i].front()]; ^ user.cpp:28:21: warning: array subscript has type ‘char’ [-Wchar-subscripts] ch[words[i].back()]=i; ^