public ListNode Merge(ListNode list1,ListNode list2) {

        ListNode head1 = list1;

        ListNode head2 = list2;

        ListNode newHead = new ListNode(-1);

        ListNode ptr = newHead;

        while (head1!=null&&head2!=null){

            if(head1.val<=head2.val){

                ptr.next = head1;

                head1 = head1.next;

            }else{

                ptr.next = head2;

                head2 = head2.next;

            }

            ptr = ptr.next;

        }

        if(head1==null)

            ptr.next = head2;

        else

            ptr.next = head1;

        return newHead.next;

    }