【算法】合并两个排序的链表

public ListNode Merge(ListNode list1, ListNode list2) {
        // 新建一个头节点，用来存合并的链表。
        ListNode head = new ListNode(-1);
        head.next = null;
        ListNode root = head;
        while (list1 != null && list2 != null) {
            if (list1.val < list2.val) {
                head.next = list1;
                head = list1;
                list1 = list1.next;
            } else {
                head.next = list2;
                head = list2;
                list2 = list2.next;
            }
        }
        // 把未结束的链表连接到合并后的链表尾部
        if (list1 != null) {
            head.next = list1;
        }
        if (list2 != null) {
            head.next = list2;
        }
        return root.next;
    }