Compilare: user.cpp: In function ‘int f()’: user.cpp:12:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0;i<strlen(S);++i) ^ user.cpp:19:18: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] a[i-j-1]=NULL; ^ user.cpp:24:18: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] b[j-i-1]=NULL; ^ user.cpp: In function ‘int main()’: user.cpp:37:20: warning: format ‘%s’ expects argument of type ‘char*’, but argument 2 has type ‘char (*)[1001]’ [-Wformat=] scanf("%s",&S); ^