Compilare: main.cpp: In function 'void add(trie*, char*)': main.cpp:13:16: warning: NULL used in arithmetic [-Wpointer-arith] 13 | if ( *s == NULL ) | ^~~~ main.cpp: In function 'void del(trie*, char*)': main.cpp:24:16: warning: NULL used in arithmetic [-Wpointer-arith] 24 | if ( *s == NULL ) | ^~~~ main.cpp: In function 'int freq(trie*, char*)': main.cpp:33:16: warning: NULL used in arithmetic [-Wpointer-arith] 33 | if ( *s == NULL ) | ^~~~ main.cpp: In function 'int lcp(trie*, char*, int)': main.cpp:41:16: warning: NULL used in arithmetic [-Wpointer-arith] 41 | if ( *s == NULL ) | ^~~~