Compilare: user.cpp: In function ‘int main()’: user.cpp:20:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < a.length(); ++i) { ^ user.cpp:24:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < b.length(); ++i) { ^ user.cpp:44:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (z[i] >= a.length()) { ^