Compilare: user.cpp: In function ‘bool sgn(char)’: user.cpp:14:27: warning: NULL used in arithmetic [-Wpointer-arith] if(c=='-'||c=='+'||c==NULL||c=='=') ^ user.cpp: In function ‘int main()’: user.cpp:61:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for ( i = j ; i < strlen(c) ; i ++ ) ^