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.
 Example:
 Input: 1->2->4, 1->3->4
 Output: 1->1->2->3->4->4 

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) 
{
    if(NULL == l1 && NULL != l2)
        return l2;
    else if(NULL == l2 && NULL != l1)
        return l1;
    else if (NULL == l1 && NULL == l2)
        return NULL;
    
    struct ListNode *head = NULL;
    /*返回新链表的头结点*/
    struct ListNode *ret = NULL;

    /*确定链表头*/
    if(l1->val < l2->val)
    {
        head = l1;
        l1 = l1->next;
    }
    else
    {
        head = l2;
        l2 = l2->next;
    }
    
    /*保存链表头*/
    ret = head;
    
    while(l1 &&l2)
    {
        if(l1->val >l2->val)
        {
            head->next = l2;
            l2 = l2->next;
        }
        else
        {
            head->next = l1;
            l1 = l1->next;
        }
        /*新插入的结点next置NULL*/
        head->next->next = NULL;
        head = head->next;
            
    }
    
    if(l1)
        head->next = l1;
    else if (l2)
        head->next = l2;
    
    return ret;
}