Compilare: user.cpp: In function ‘void build_prefix()’: user.cpp:22:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 1; i < a.length(); i++) ^ user.cpp:35:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i = 0; i < a.length(); i++) ^ user.cpp: In function ‘int main()’: user.cpp:52:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < b.length(); i++) ^