Sheet 3

public class PracticeSheet3 {
    static class ListNode {
        int val;
        ListNode next;

        ListNode(int val) {
            this.val = val;
            this.next = null;
        }
    }

    static class SwapNodes {
        public ListNode swapNodes(ListNode head, int position1, int position2) {
            if (head == null || position1 == position2) {
                return head;
            }

            ListNode prev1 = null, curr1 = head;
            ListNode prev2 = null, curr2 = head;

            // Traverse the list to find the nodes to swap
            for (int i = 1; curr1 != null && i < position1; i++) {
                prev1 = curr1;
                curr1 = curr1.next;
            }
            for (int i = 1; curr2 != null && i < position2; i++) {
                prev2 = curr2;
                curr2 = curr2.next;
            }

            // If one or both positions are out of bounds, return the original head
            if (curr1 == null || curr2 == null) {
                return head;
            }

            // If the nodes to swap are not adjacent
            if (prev1 != null && curr2.next != curr1) {
                prev1.next = curr2;
            } else if (prev1 == null) { // If the first position is the head
                head = curr2;
            }

            if (prev2 != null && curr1.next != curr2) {
                prev2.next = curr1;
            } else if (prev2 == null) { // If the second position is the head
                head = curr1;
            }

            // Swap the next pointers of the nodes to perform the swap
            ListNode temp = curr1.next;
            curr1.next = curr2.next;
            curr2.next = temp;

            return head;
        }

        // Helper method to print the linked list
        public void printList(ListNode head) {
            ListNode current = head;
            while (current != null) {
                System.out.print(current.val + " ");
                current = current.next;
            }
            System.out.println();
        }

    }
    public static void main(String[] args) {
        SwapNodes swap = new SwapNodes();

        // Example linked list: 1 -> 2 -> 3 -> 4 -> 5
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = new ListNode(4);
        head.next.next.next.next = new ListNode(5);

        System.out.println("Original linked list:");
        swap.printList(head);

        // Swap nodes at positions 2 and 4
        head = swap.swapNodes(head, 2, 4);

        System.out.println("Linked list after swapping nodes:");
        swap.printList(head);
    }
}

//output
Original linked list:
1 2 3 4 5 
Linked list after swapping nodes:
1 4 3 2 5