【leetcode】合并两个排序的链表

struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2){
    if (l1 == NULL && l2 == NULL)
        return NULL;
    else if(l1 == NULL)
        return l2;
    else if(l2 == NULL)
        return l1;
    struct ListNode* head;
    struct ListNode* tail;
    if (l1->val <= l2->val) // 找出两个链表第一个值最小值选择做头部开始
    {
        head = l1;
        tail = l2;
    }
    else
    {
        head = l2;
        tail = l1;
    }
    struct ListNode* ret = head; // 用来返回的指针
    struct ListNode* p; // 循环里用来转换的
    while(head->next!=NULL && tail!=NULL)
    {
        if (tail->val <= head->next->val)
        {
            p = tail->next;
            tail->next = head->next;
            head->next = tail;
            tail = p;
            head = head->next;
        }
        else
            head = head->next;
    }
    if (head->next == NULL)
    {
        head->next = tail;
    }
    return ret;
}