双指针法

    public static ListNode Merge(ListNode list1, ListNode list2) {
        if (list1 == null && list2 == null) {
            return null;
        } else if (list1 == null) {
            return list2;
        } else if (list2 == null) {
            return list1;
        }
        ListNode p1 = list1;
        ListNode p2 = list2;
        ListNode dumpy = new ListNode(-1);
        ListNode curr = dumpy; // 将当前指针指向头节点
        dumpy.next = p1.val < p2.val ? p1 : p2;
        while (p1 != null && p2 != null) {
            if (p1.val <= p2.val) {
                curr.next = p1;
                p1 = p1.next;
            } else {
                curr.next = p2;
                p2 = p2.next;
            }
            curr = curr.next; // 当前指针向后移动
        }
        // 注意：如果p1为空，则最后一个元素为p2，反之亦然
        curr.next = p1 != null ? p1 : p2;
        return dumpy.next;
    }