Compilare: user.cpp: In function ‘int main()’: user.cpp:20:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i=0;i<strlen(S);i++) ^ user.cpp:27:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i=0;i<strlen(S);i++) ^ user.cpp:43:9: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] t[k]=NULL; ^