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Henry Wang
Henry Wang
6,412 Points

Swapping Nodes of a LinkedList

I am trying to swap the adjacent nodes of a Linked List, but when I test my code using [] as input, I am getting NullPointerException instead of runtime error (which is what the online judge wants).

Could someone please take a look at my code and see what's going wrong?


 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
class Solution {
    public ListNode swapPairs(ListNode node) {

        if (node.next == null) {
            return node;

        if (node == null) {


        return node;

    public static void swap(ListNode node) {
        ListNode temp = new ListNode(node.val);
        temp.next = node.next;

        ListNode tempNext = new ListNode(node.next.val);
        tempNext.next = node.next.next;

        // Begin Swapping
        node.val = node.next.val;
        node.next = temp;

        if (tempNext == null) {
            node.next.next = null;

        } else {
            temp.next = tempNext.next;


        if (node.next.next != null) {


Steven Parker
Steven Parker
229,968 Points

Can you provide a link to the page (with the "online judge")?

1 Answer

Steven Parker
Steven Parker
229,968 Points

This doesn't look like it would compile. The "return" on line 17 is missing both the return value and the semicolon to end the statement.

If that's not the whole issue, please make a snapshot of your workspace and post the link to it here. Or post a link to the course page if it's from a challenge.