Compilare: main.cpp: In function 'bool pair_up(int)': main.cpp:26:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i = 0; i < a[x].size(); i++) ~~^~~~~~~~~~~~~ main.cpp:32:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i = 0; i < a[x].size(); i++) ~~^~~~~~~~~~~~~ main.cpp: In function 'void cover_up(int)': main.cpp:44:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i = 0; i < a[x].size(); i++) ~~^~~~~~~~~~~~~