Compilare: main.cpp: In function 'int main()': main.cpp:18:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i<a.length(); i++) ~^~~~~~~~~~~ main.cpp:24:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i<a.length(); i++) ~^~~~~~~~~~~ main.cpp:35:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i<v1.size(); i++) ~^~~~~~~~~~