Compilare: user.cpp: In function ‘void insert_word(trie*, char*)’: user.cpp:23:13: warning: NULL used in arithmetic [-Wpointer-arith] if (*s==NULL) ^ user.cpp: In function ‘int word_frequency(trie*, char*)’: user.cpp:39:13: warning: NULL used in arithmetic [-Wpointer-arith] if (*s==NULL) ^ user.cpp: In function ‘int longest_common_prefix(trie*, char*)’: user.cpp:49:13: warning: NULL used in arithmetic [-Wpointer-arith] if (*s==NULL) ^ user.cpp: In function ‘bool erase_word(trie*, char*)’: user.cpp:59:13: warning: NULL used in arithmetic [-Wpointer-arith] if (*s==NULL) ^ user.cpp: In function ‘int word_frequency(trie*, char*)’: user.cpp:45:1: warning: control reaches end of non-void function [-Wreturn-type] } ^