# Reversing a singly linked list

#### jonas

##### Well-Known Member
Code:
``````rec_reverse(last_of_rev,to_rev) = if to_rev != nil:
next_rev = to_rev.next;
to_rev.next = last_of_rev;
rec_reverse(to_rev,next_rev)

reverse(to_rev) = rec_reverse(nil, to_rev)``````
Code:
``````reversed list    list to be reversed
(|<-()<-...<-()  ,  []->()->...->()->|)
after one recurisve step becomes:
(|<-()<-...<-()<-[]  ,  ()->...->()->|)
at the end of recursion becomes:
(|<-()<-...<-()<-[]<-()<-...<-()  ,  |)``````

Last edited:

#### Accname

##### 2D-Graphics enthusiast
Code:
``````    public void reverse() {
Node prev = null;
Node current = first;
while (current != null) {
Node next = current.next;
current.next = prev;
prev = current;
current = next;
}
first = prev;
}``````
This is using a simple while-loop.
Here a full example with test case:
Code:
``````  public static void main(final String[] args) {
LList l = new LList();
l.first = new Node("A");
l.first.next = new Node("B");
l.first.next.next = new Node("C");
l.first.next.next.next = new Node("D");
l.first.next.next.next.next = new Node("E");
l.first.next.next.next.next.next = new Node("F");

System.out.println(l);
System.out.println();
l.reverse();
System.out.println(l);
System.out.println();
}

public static class LList {
Node first;

public void reverse() {
Node prev = null;
Node current = first;
while (current != null) {
Node next = current.next;
current.next = prev;
prev = current;
current = next;
}
first = prev;
}

@Override
public String toString() {
StringBuilder sb = new StringBuilder();
Node current = first;
while (current != null) {
sb.append(current.value);
sb.append(" ");
current = current.next;
}
return sb.toString();
}
}

static class Node {
Object value;
Node next;

public Node(Object value) {
this.value = value;
}

}``````
Output:
A B C D E F

F E D C B A

#### jonas

##### Well-Known Member
it's quite much literally the same (minus a bug in my code)

#### Accname

##### 2D-Graphics enthusiast
Its not quite the same depending on the execution enviroment and language you are using. A loop is much better in java than recursion. Sure, at some point the JIT will jump in and might improve the recursive method, but until then the loop will outperform the recursion every time.

#### jonas

##### Well-Known Member
Its not quite the same depending on the execution enviroment and language you are using. A loop is much better in java than recursion. Sure, at some point the JIT will jump in and might improve the recursive method, but until then the loop will outperform the recursion every time.
Note that I was not giving a java program... At the conceptual level the two are identical, and that was the level in which I was giving the algorithm. Yes, in a given actual implementation the performance between the two may differ significantly, and one of them may run out of memory while the other may not (this is actually the bigger issue). From my point of view, you have simply given the idiomatic Java implementation of the conceptual algorithm above. Maybe we can find common ground here?

#### Accname

##### 2D-Graphics enthusiast
You have to go to a veeeeerrrryyy high conceptual level to call these two implementations equal. Sure, in the end they do the same thing because thats what they are supposed to do. But both use different syntactic and semantic constructs some of which may or may not be available in a given language / runtime enviroment.

#### jonas

##### Well-Known Member
You have to go to a veeeeerrrryyy high conceptual level to call these two implementations equal. Sure, in the end they do the same thing because thats what they are supposed to do. But both use different syntactic and semantic constructs some of which may or may not be available in a given language / runtime enviroment.
The first one is not an implementation, it's a description of the algorithm. There might not even be a compiler or interpreter for that language.
I wouldn't just say that they do the same thing, I'd go as far to say they do it the *same way*, namely by going through the pointers and flipping them one by one. Recursion and iteration just say "repeatedly do this", or "repeatedly do this until".

#### Accname

##### 2D-Graphics enthusiast
Thats a little to vague for my taste. A loop is an actual syntactical structure defined by a language. Recursion is a method of software-developement that can be used in most languages but there is no syntactical component to it in any language I know of. There are many different languages, some of which do not have loops. Some of them do not have the concept of methods either but they have loops. Some have methods but do not allow for recursion, etc.
I can not say with a straight face that these two things are "the same". They might be very similar, they might be exchangeable in most enviroments, but they are not "the same".

#### jonas

##### Well-Known Member
Well, I understand that you live in the practical world of building real software, where there are very clearly specific definitions and precise differences for these definitions. If I would develop a Java program, I would use a while loop, not a recursive method call, just like you have.

I on the other hand live in the theoretical world of building language models and proving that can be compiled correctly to run on real machines, so I'm looking more at the conceptual level of what languages can do (in general) and how it would be compiled and run, and so in my world they use the same mechanisms and I don't distinguish between them. Whether a language implements "doing the same thing over and over" by function recursion or loop recursion or a recursion operator (like system T) is completely irrelevant to me; if it has a way of "doing the same thing over and over", it needs to do a) b) and c) to compile correctly for x86.

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