Method merge

java/0148-sort-list.java:21–42

(ListNode left, ListNode right)

        return slow;
    }
    public ListNode merge(ListNode left, ListNode right){
        ListNode dummy = new ListNode();
        ListNode tail = dummy;

        while(left != null && right != null){
            if(left.val < right.val){
                tail.next = left;
                left = left.next;
            }else{
                tail.next = right;
                right = right.next;
            }
            tail = tail.next;
        }
        if(left != null){
            tail.next = left;
        }
        if(right != null){
            tail.next = right;
        }
        return dummy.next;
    }
    public ListNode sortList(ListNode head) {
        if(head == null || head.next == null){
            return head;