Merge Sort Linked List

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        
        ListNode slow = head;
        ListNode fast = head;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        
        ListNode head1 = head;
        ListNode head2 = slow.next;
        slow.next = null;
        head1 = sortList(head1);
        head2 = sortList(head2);
        return merge(head1, head2);
    }
    
    private ListNode merge(ListNode head1, ListNode head2) {
        if (head1 == null) {
            return head2;
        }
        
        if (head2 == null) {
            return head1;
        }
        
        ListNode dummyNode = new ListNode(0);
        ListNode curr = dummyNode;
        while (head1 != null && head2 != null) {
            if (head1.val <= head2.val) {
                curr.next = head1;
                head1 = head1.next;
            } else {
                curr.next = head2;
                head2 = head2.next;
            }
            
            curr = curr.next;
        }
        
        curr.next = head1 == null ? head2 : head1;
        return dummyNode.next;
    }
}