1. Recursive Linked Lists
Lecture 29
Sections 10.3 – 10.4
Wed, Apr 2, 2008

2. Recursive Linked Lists
A linked list is a naturally recursive structure.
The “linked list” is a pointer to a node. (Ignore the mSize data member.)
Furthermore, each node contains a pointer to a node.
Therefore, each node contains a “linked list.”
However, the sublists do not have an mSize data member, so they are not exactly LinkedList objects.
5/4/2019
Recursion

3. Two-Part Implementation of Recursive Member Functions
We implement the recursive functions in two parts.
The first function is public and non-recursive.
Type Class::Function(parameters) {
// Do some special things…
// Call the second function,
// passing the head of the list
Function(head, parameters);
// Do more special things…
return; }
5/4/2019
Recursion

4. Implementation of Recursive Member Functions
The second function is private and recursive.
Type Class::Function(LinkedListNode* node, parameters) {
if (condition)
// Handle the recursive case
return Function(node->next, parameters); }
else
// Handle the non-recursive case
5/4/2019
Recursion

5. Example: The insert() Function
There are two things that the non-recursive insert() function should do only once.
Check that pos is valid.
Increment mSize.
Then it makes a call to the recursive insert() function, passing it the head pointer.
5/4/2019
Recursion

6. The Non-Recursive insert() Function
void insert(int pos, const T& value) {
// Do something special
assert(pos >= 1 && pos <= mSize);
// Call the recursive function
insert(head, pos, value);
mSize++;
return; }
5/4/2019
Recursion

7. The Recursive insert() Function
The recursive insert() function must distinguish two cases.
Recursive case
The insertion takes place at a later node.
Non-recursive case
The insertion takes place at the current node, which is the “head” of the (sub)list.
5/4/2019
Recursion

8. The Recursive Case The recursive case
Is distinguished by the condition that pos > 0 (a later node).
Passes a pointer to the next node.
Decrements the value of pos.
insert(node->next, pos – 1, value);
5/4/2019
Recursion

9. The Non-Recursive Case
Is distinguished by the condition pos == 0 (this node).
Inserts the new node at the head of the (sub)list.
Because node is modified, it must be passed by reference.
LinkedListNode<T>* new_node =
new LinkedListNode<T>(value);
new_node->next = node;
node = new_node;
5/4/2019
Recursion

10. The Recursive insert() Function
void insert(LinkedListNode<T>*& node, int pos,
const T& value) {
// Recursive case
if (pos > 1)
Insert(node->next, pos – 1, value);
// Non-recursive case
else
LinkedListNode<T>* new_node =
new LinkedListNode<T>(value);
new_node->next = node;
node = new_node; }
return;
5/4/2019
Recursion

11. Recursive Linked List Implementation
Example
recurlinkedlist.h
ListTest.cpp
5/4/2019
Recursion

12. Evaluation of Lists Type of List Get Element (seq) (rand) Insert
Delete
Push
Front
Pop
Back
Array List
0.020
0.116
39.5
40.9
50.4
47.8
0.035
0.030
Circ Array List
0.015
0.074
12.3
12.2
0.027
0.032
Linked List
23.2
21.5
47.0
57.9
0.183
0.144
52.4
99.2
Linked List w Tail
23.3
21.7
57.1
0.178
0.143
0.163
52.3
Doubly Linked List
13.1
12.7
25.0
30.1
0.171
0.121
0.165
Circ Linked List
13.0
12.8
24.7
29.7
0.168
0.115
0.117
Recur Linked List
53.5
49.3
89.4
91.4
0.132
148
135
5/4/2019
Recursion