Printing out elements of a linked list in reverse

Question

I am practicing my data structures and trying out this problem on HackerRank on printing the element of a linked list in reverse order.

I have looked through my solutions multiple times but I don't understand why it is wrong.

My Solution:

    static void reversePrint(SinglyLinkedListNode head) {
        if (head==null){
            return;
        }

        ArrayList<Integer> intList = new ArrayList<>();

        while(head!=null){
            intList.add(head.data);
            head = head.next;
        }

         Collections.sort(intList, Collections.reverseOrder());
        for(int i: intList){
            System.out.println(i);
        }
    }

Appreciate if someone can assist on pointing out my mistakes.

Answer 1

Use Stack rather than ArrayList

static void reversePrint(SinglyLinkedListNode head) {
    if (head==null){
        return;
    }

    Deque<Object> intList = new ArrayDeque<>();
    while(head!=null){
        intList.push(head.data);
        head = head.next;
    }

    while(!intList.isEmpty()){
        System.out.println(intList.pop());
    }
}

Answer 2

Appreciate if someone can assist on pointing out my mistakes

The Problem

You used an ArrayList whose behavior is F irst I n F irst O ut , so the order of the elements printed when you for -loop the intList is the same as the order in the linked list, starting from head .

Possible solutions

To fix your approach, you could simply:

1- Insert each element always in the position zero of intList using List#add(int index, E element) :

// ...
intList.add(0, head.data);
// ...

2- Or, you can replace the data structure you used to "revert" the elements.

For example, you can use any of the L ast I n F irst O ut data structures, such as Queue or Deque in their LIFO variant.

---

Another alternative

Giving you another completely different solution, you could implement it in a recursive manner like this:

static void reversePrint(SinglyLinkedListNode head) {
    if (head != null) {
        reversePrint(head.next);
        System.out.println(head.data);
    }
}

If head is null , do not print anything. Otherwise, first reversePrint the next element and after that, the current one ( head.data )... this will cause the reversal effect.

It's worth noting that this doesn't reverse the order of elements, that is, it doesn't modify the next of each element.