Compilare: main.cpp: In function 'int main()': main.cpp:11:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for ( l = 1 ; l <= a.size() / 3 ; ++ l ) { ~~^~~~~~~~~~~~~~~ main.cpp:12:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for ( i = 0 ; i < a.size() - 3*l + 1 ; ++ i ) { ~~^~~~~~~~~~~~~~~~~~~~