Answer to Question linkedlist3-2

  void addToEnd(int x, List& L)
  {
    if(isEmpty(L))
    {
      L = cons(x, emptyList);
    }
    else
    {
      addToEnd(x, L->next);
    }
  }

Since the definition of addToEnd is tail-recursive, an optimizing compiler can turn it into a loop. You can write it directly as a loop using a reference variable, which refers to another variable.

  void addToEnd(int x, List& L)
  {
    List& p = L;
    while(!isEmpty(p))
    {
      p = p->next;
    }
    p->next = cons(x, emptyList);
  }

When the loop ends, p is a variable that holds NULL. It is the 'next' variable in the last cell, or is variable L if L is NULL. It is important to use p->next instead of tail(p) since tail(p) is not a variable.