OddEvenLinkedList.java

package LeetcodePractice;

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

    
    public static 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;
        return head;
    }

    
    public static ListNode createList(int[] values) {
        ListNode dummy = new ListNode(0);
        ListNode current = dummy;
        for (int val : values) {
            current.next = new ListNode(val);
            current = current.next;
        }
        return dummy.next;
    }

   
    public static void printList(ListNode head) {
        while (head != null) {
            System.out.print(head.val);
            if (head.next != null) System.out.print(" -> ");
            head = head.next;
        }
        System.out.println();
    }

  
    public static void main(String[] args) {
        int[] input = {1, 2, 3, 4, 5}; 
        ListNode head = createList(input);

        System.out.print("Original List: ");
        printList(head);

        head = oddEvenList(head);

        System.out.print("Reordered List: ");
        printList(head);
    }
}