Merge Two Sorted Lists

题目描述：

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.

　　直接贴答案了。

solution：

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
    if(l1 == NULL)
        return l2;
    if(l2 == NULL)
        return l1;
    ListNode *head = new ListNode(0);
    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;
    }
    p->next = l1 ? l1 : l2;
    return head->next;
}

 递归解法：

ListNode* mergeTwoLists(ListNode *l1, ListNode *l2)
{
    if(l1 == NULL)
        return l2;
    if(l2 == NULL)
        return l1;
    if (l1->val < l2->val)
    {
        l1->next = mergeTwoLists(l1->next, l2);
        return l1;
    }
    else
    {
        l2->next = mergeTwoLists(l2->next, l1);
        return l2;
    }
}