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

public class Solution {
    public ListNode oddEvenList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        
        ListNode odd = head;
        ListNode even = head.next;
        ListNode evenHead = even;
        
        while (even != null && even.next != null) {
            odd.next = even.next;
            odd = odd.next;
            even.next = odd.next;
            even = even.next;
        }
        
        odd.next = evenHead; // Link the end of odd list to the even list head
        return head;
    }
}