問題描述：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
    if(l1 == NULL)
    {
        return l2;
    }
    else if(l2 == NULL)
    {
        return l1;
    }
    
    //先取小的節點作為頭
    struct ListNode* head, *tail;
    if(l1->val < l2->val)
    {
        head = l1;
        l1 = l1->next;
    }
    else
    {
        head = l2;
        l2 = l2->next;
    }
    
    //拿小的節點進行尾插
    tail = head;
    while(l1 !=NULL && l2 != NULL)
    {
        if(l1->val < l2->val)
        {
            tail->next = l1;
            l1 = l1->next;
        }
        else
        {
            tail->next = l2;
            l2 = l2->next;
        }
        tail = tail->next;
    }
    
    if(l1 != NULL)
    {
        tail->next = l1;
    }
    
    if(l2 != NULL)
    {
        tail->next = l2;
    }
    return head;
}