Compilare: user.cpp: In function 'int main()': user.cpp:40: warning: comparison between signed and unsigned integer expressions user.cpp:48: warning: comparison between signed and unsigned integer expressions user.cpp: In function 'void back(int)': user.cpp:93: warning: comparison between signed and unsigned integer expressions user.cpp:105: warning: comparison between signed and unsigned integer expressions user.cpp: In function 'void sort(char*)': user.cpp:125: warning: right-hand operand of comma has no effect user.cpp:126: warning: comparison between signed and unsigned integer expressions