Compilare: user.cpp: In function ‘int main()’: user.cpp:17:17: warning: NULL used in arithmetic [-Wpointer-arith] while(s[i]!=NULL) ^ user.cpp:19:72: warning: NULL used in arithmetic [-Wpointer-arith] while(!((s[i]>='a'&&s[i]<='z')||(s[i]>='A'&&s[i]<='Z'))&&s[i]!=NULL) i++; ^ user.cpp:22:71: warning: NULL used in arithmetic [-Wpointer-arith] while(((s[i]>='a'&&s[i]<='z')||(s[i]>='A'&&s[i]<='Z'))&&s[i]!=NULL) ^