21.合并两个有序链表

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ListNode dummy = new ListNode(-1);
        
        
        ListNode cur = dummy;
        while (list1 != null && list2 != null) {
            if (list1.val <= list2.val) {
                cur.next = list1;   
                list1 = list1.next; 
            } else {
                cur.next = list2;   
                list2 = list2.next; 
            }
            
            cur = cur.next;
        }

        if (list1 != null) {
            cur.next = list1;
        } else if (list2 != null) {
            cur.next = list2;
        }

        return dummy.next;