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.

代码：

ListNode * Solution::mergeTwoLists(ListNode *l1, ListNode *l2)
{
    ListNode * newList = NULL;
    ListNode * rear = NULL;
    while(l1 != NULL && l2 != NULL)
    {
        if(l1->val < l2->val)
        {
            insertIntoList(&newList,&rear,l1->val);
            l1 = l1->next;
        }
        else
        {
            insertIntoList(&newList,&rear,l2->val);
            l2 = l2->next;
        }
    }

    while(l1 != NULL)
    {
            insertIntoList(&newList,&rear,l1->val);
            l1 = l1->next;
    }

    while(l2 != NULL)
    {
            insertIntoList(&newList,&rear,l2->val);
            l2 = l2->next;
    }
    return newList;
}

void Solution::insertIntoList(ListNode ** newList,ListNode ** rear,int val)
{
            ListNode * newNode = (ListNode *)malloc((sizeof(ListNode)));
            newNode->val = val;
            newNode->next = NULL;
            if((*newList) == NULL)
            {
                (*newList) = newNode;
                (*rear)    = newNode;
            }
            else
            {
                (*rear)->next = newNode;
                (*rear) = (*rear)->next;
            }
}

LeetCode:Merge Two Sorted Lists