Discussion on Reverse a doubly linked list Challenge

svadali2 3 years ago+ 1 comment

Hey could you explain why this works? because I put a print statement right at the end of the loop after "temp = temp.prev" and it gave me a nullpointer exception. However, when i put that print statement at the start of loop, there was no error; could you give some sort of explanation of this algorithm? thanks.

gswheeler 3 years ago+ 1 comment

It works like this: in a double-linked list that has been reversed, the values of each node's 'next' and 'prev' fields are swapped (the nodes before and after each node swap positions). The code above does just that; starting at the head of the list it works through the list swapping 'next' and 'prev' values.

As for why you got a NullPointer exception, a pointer without a target has a value of NULL, thus a loop that moves from node to node ends when its "iterator" variable is assigned null (that's why newHead's value is updated before temp's value). In short, your print statement tried to get a field value from said null.

cmveal 1 month ago+ 0 comments

Here's a four line solution using recursion.

    static DoublyLinkedListNode reverse(DoublyLinkedListNode curr) {
    DoublyLinkedListNode temp = curr.next;
    curr.next = curr.prev;
    curr.prev = temp;
    return temp == null ? curr : reverse(temp);
}