Compilare: main.cpp: In function 'int main()': main.cpp:51:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for( i = 0; i < a.size(); ++i ) ~~^~~~~~~~~~ main.cpp:54:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for( i = 0; i < b.size() && !impossible; ++i ){ ~~^~~~~~~~~~ main.cpp:63:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i = 0; i < a.size(); ++i ) { ~~^~~~~~~~~~