Reverse linked list in java

Question

I would like to know if this is a valid recursive solution and if any improvements can be made. I've tested this solution with a few inputs and it seems to work.

public Node reverse(Node head) {

if(head == null) {
    return null;
}

if (head.next == null) {
    this.head = head;
    return head;
}

Node n = head.next;
head.next = null;   

reverse(n).next = head; 

return head;
}

Thank you.

Answer 1

In terms of algorithm, I think this method does what you want, I believe you wanted a linked list

A->B->C

to be converted to

C->B->A, and return A.

I am just a little confused with the scope of this method, is it a instance method of Node? what "this" is at line 8.

One thing I am not sure is, if you do not keep other pointers to C and B, after the reverse is done, then you are left with the list C->B->A, with a returned pointer to A. It looks like C and B are not pointed by any variables, won't it be garbage collected? and removed? Maybe this problem is solved by the "this" thing.

A good practice of constructing a list should be a method that return the head of a list. But your method returns the tail. I suggest make this an instance method of Node, then you make the reverse() method to be called with head, and return the new head

public Node reverse() {

    if (this.next == null) {    
        return this;
    }

    Node n = this.next;
    Node newhead=n.reverse()  // After reverse, n should be at the tail 
    n.next = this; 

    return newhead;
}

something like this, but I have not tested the code