Compilare: user.cpp: In constructor ‘huffman::huffman()’: user.cpp:18:62: warning: converting to non-pointer type ‘long long unsigned int’ from NULL [-Wconversion-null] huffman() : lf(NULL), rg(NULL), is_leaf(true), times(NULL) {} ^ user.cpp: In function ‘int main()’: user.cpp:71:49: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] else if(qf.front() <= qn.front() -> times){ ^ user.cpp:97:49: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] else if(qf.front() <= qn.front() -> times){ ^