Compilare: user.cpp: In function ‘int valid(int)’: user.cpp:12: warning: comparison between signed and unsigned integer expressions user.cpp: In function ‘void back(int)’: user.cpp:20: warning: comparison between signed and unsigned integer expressions user.cpp:23: warning: comparison between signed and unsigned integer expressions user.cpp:22: warning: suggest explicit braces to avoid ambiguous ‘else’ user.cpp: In function ‘int main()’: user.cpp:35: warning: operation on ‘i’ may be undefined user.cpp:39: warning: comparison between signed and unsigned integer expressions