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33C. Example: Remove a Value from a Linked List

Suppose that remove(x, L) is intended to be a destructive function that removes the first occurrence of x from list L, or does nothing if x does not occur in L. For example, if L is [2, 4, 2, 4, 5] and you perform remove(4, L), then L is changed to [2, 2, 4, 5]. The cases are as follows.

  1. If L is empty, do nothing, since x obviously does not occur in L.

  2. If L is nonempty and the head of L is equal to x, then set L to its tail. However, the cell that contains x is presumably no longer needed. So it should be deleted. Look at how it needs to work when L is [2, 4, 6] and x is 2.

  3. If L is not empty and its head is not equal to x, then remove x from the tail of L.

Expressing that in C++ yields the following definition of remove. In the second case, we need to be careful manipulating pointers to avoid cutting the branch out from under ourselves. 
  void remove(const int x, List& L)
  {
    if(L != NULL))
    {
      if(L->head == x)
      {
        List p = L;
        L = L->tail;
        delete p;
      }
      else
      {
        remove(x, L->tail))
      }
    }
  }

 

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