将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的

public:

ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
    ListNode *result = new ListNode(-1); //哑节点简化代码
    ListNode *workNode = result;
    while (l1 != nullptr && l2 != nullptr)
    {
        if (l1->val <= l2->val)
        {
            workNode->next = l1;
            l1 = l1->next;
        }
        else
        {
            workNode->next = l2;
            l2 = l2->next;
        }
        workNode = workNode->next;
    }
    workNode->next = l1 != nullptr ? l1 : l2;

    return result->next;
}