Compilare: user.cpp: In function ‘int main()’: user.cpp:24:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (index_first != first.size() - 1) { ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~ user.cpp:31:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (index_second == second.size()) { ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~