infinite loop, LeetCode 1721

Question

here's my code:

class Solution {
    public int size (ListNode head)
    {
        int size = 0;
        ListNode curr = head;
        while (curr != null)
        {
            size ++;
            curr = curr.next;
        }
        return size;
    }
    public ListNode swapNodes(ListNode head, int k) {
        ListNode curr = head;
        ListNode curr2 = head;
        ListNode pre = null;
        ListNode pre2 = null;
        ListNode result = head;
        for (int i = 1; i < k && curr != null; i ++)
        {
            pre = curr;
            curr = curr.next;
        }
        int size = size(head);
        for (int i = 0; i < (size - k) && curr2 != null; i ++)
        {
            pre2 = curr2;
            curr2 = curr2.next;
        }
        ListNode tmp = curr.next;
        ListNode tmp2 = curr2.next;
        pre.next = curr2;
        curr2.next = tmp;
        pre2.next = curr;
        curr.next = tmp2;
        return head;
        
    }
}

there seems to be an infinite loop in one of the cases and i can't fix it, can someone help please?

Answer 1

for me everything works fine. Maybe you are using other inputs or initialising the List in a wrong way.

I added a method to your ListNode class to create the list in a fluent way:

public ListNode addListNode(int v) {
    ListNode listNode = new ListNode(v);
    next = listNode;
    return listNode;
}

and a print Method to your Solution class to print the list.

public static void printList(ListNode head)
{
    StringBuilder out = new StringBuilder();
    ListNode curr = head;
    while (curr != null)
    {
        out.append(curr.val).append(",");
        curr = curr.next;
    }
    System.out.println(out);
}

Running it with this main method:

public static void main(String[] args) {
    ListNode listNode = new ListNode(1);
    ListNode currNode = listNode.addListNode(2);
    for( int i = 3; i <= 10; i++) {
        currNode = currNode.addListNode(i);
    }
    // you also could write listNode.addListNode(2).addListNode(3).addListNode(4) ...

    printList(listNode);
    System.out.println("SwapNodes");
    ListNode swappedListNode = swapNodes(listNode, 2);
    printList(swappedListNode);
}

it outputs

1,2,3,4,5,6,7,8,9,10,
SwapNodes
1,9,3,4,5,6,7,8,2,10,

So it seems to work as expected, no infinite loop. As I mentioned at the beginning feel free to add your main method and the way you are calling the swapNodes Method.

You can edit your question and add these informations..