First, I don't think int B[10] = { NULL };
is good method for whatever you wanted.
If you just want do the postorder traversal, the easier way to implement is:
void postTraversal(int a[], int n)
{
if (n <=9 && n >= 0)
{
postTraversal(a, L(n));
postTraversal(a, R(n));
cout << a[n] << " ";
}
}
int main()
{
int A[10]={0,1,2,3,4,5,6,7,8,9};
//f(A,0);
postTraversal(A, 0);
cout << endl;
return 0;
}