Compilare: user.cpp: In function ‘int valid(int)’: user.cpp:15: warning: comparison between signed and unsigned integer expressions user.cpp: In function ‘void back(int)’: user.cpp:26: warning: comparison between signed and unsigned integer expressions user.cpp:46: warning: comparison between signed and unsigned integer expressions user.cpp:45: warning: suggest explicit braces to avoid ambiguous ‘else’ user.cpp:55: warning: comparison between signed and unsigned integer expressions user.cpp:55: warning: suggest explicit braces to avoid ambiguous ‘else’ user.cpp: In function ‘int main()’: user.cpp:86: warning: comparison between signed and unsigned integer expressions