Compilare: main.cpp: In function 'int main()': main.cpp:22:22: warning: converting to non-pointer type 'char' from NULL [-Wconversion-null] reff[k+1]=NULL; ^~~~ main.cpp:37:27: warning: converting to non-pointer type 'char' from NULL [-Wconversion-null] reff[k+1]=NULL; ^~~~ main.cpp:52:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if((it.first).size()>max1) max1=(it.first).size(); ~~~~~~~~~~~~~~~~~^~~~~