【LeetCode OJ】Merge Two Sorted Lists

题目：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.

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
    
};
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2)
{
    if (l2 == NULL)
        return l1;
    if (l1 == NULL)
        return l2;
    ListNode  *r, *head;
    head = NULL;
    if (l1->val <= l2->val)  //哪个链表的第一个节点小就把头指针指向哪个节点
    {
        head = l1;
        l1 = l1->next;
    }
    else
    {
        head = l2;
        l2 = l2->next;
    }
    r = head;
    while (l1&&l2)    //比较大小
    {
        if (l1->val <= l2->val)
        {
            r->next = l1;
            l1 = l1->next;
        }
        else
        {
            r->next = l2;
            l2 = l2->next;
        }
        r = r->next;
    }
    if (l1 == NULL)   //l1为空，直接连接l2
    {
        r->next = l2;
    }
    if (l2 == NULL)  //l2为空，直接连接l1
    {
        r->next = l1;
    }
    return head;
}