Compilare: main.cpp: In function 'int main()': main.cpp:30:12: warning: converting to non-pointer type 'char' from NULL [-Wconversion-null] s[i+1]=NULL; ^~~~ main.cpp:41:22: warning: array subscript has type 'char' [-Wchar-subscripts] v[++k]=s[j]; ^ main.cpp:43:18: warning: array subscript has type 'char' [-Wchar-subscripts] v[++k]='.'; ^ main.cpp:49:22: warning: array subscript has type 'char' [-Wchar-subscripts] v[++k]=st.top(); ^ main.cpp:65:22: warning: array subscript has type 'char' [-Wchar-subscripts] v[++k]=st.top(); ^