Compilare: main.cpp: In function 'int main()': main.cpp:20:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<a.size();i++){ ~^~~~~~~~~ main.cpp:24:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j=0;j<b.size();j++){ ~^~~~~~~~~ main.cpp:28:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int k=0;k<a.size();k++){ ~^~~~~~~~~ main.cpp:29:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int l=0;l<b.size();l++){ ~^~~~~~~~~ main.cpp:36:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int u=0;u<c.size();u++){ ~^~~~~~~~~