Leetcode -- 21. Merge Two Sorted Lists

題目：Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

思路：
本題考查基礎的鏈表操作。因爲兩個鏈表l1和l2已經有序，所以只需要按順序比較即可，把較小的結點插入到新的鏈表中。注意鏈表爲空和最後比較到鏈表尾的情況。當比較到鏈表尾部時，只需要把另一個鏈表剩下的部分直接插入到新鏈表即可。

C++代碼如下：

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    if (l1 == NULL)
        return l2;
    if (l2 == NULL)
        return l1;

    ListNode* head = NULL;

    if (l1->val < l2->val)
    {
        head = l1;
        l1 = l1->next;
    }
    else
    {
        head = l2;
        l2 = l2->next;
    }

    ListNode* p = head;
    while (l1 != NULL && l2 != NULL)
    {
        if (l1->val < l2->val)
        {
            p->next = l1;
            l1 = l1->next;
        }
        else
        {
            p->next = l2;
            l2 = l2->next;
        }
        p = p->next;
    }

    if (l1 != NULL)
        p->next = l1;
    if (l2 != NULL)
        p->next = l2;

    return head;
}