Compilare: user.cpp: In function ‘void build_prefix(std::string)’: user.cpp:25:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 1; i < a.length(); i++) ^ user.cpp:31:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 2; i < a.length(); i++) ^ user.cpp:43:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 1; i < a.length(); i++) ^ user.cpp: In function ‘int main()’: user.cpp:96:43: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(k == words[j].size() - 1) ^