题目：

输入两个递增排序的链表，合并这两个链表并使新链表中的结点仍然是按照递增排序的。例如输入下图中的链表1和链表2，则合并之后的升序链表如链表3所示。

递归：

class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)
    {
        if (pHead1 == NULL)
            return pHead2;
        if (pHead2 == NULL)
            return pHead1;

        ListNode* pMerageHead = NULL;
        if (pHead1->val <= pHead2->val)
        {
            pMerageHead = pHead1;
            pMerageHead->next = Merge(pHead1->next, pHead2);
        }
        else
        {
            pMerageHead = pHead2;
            pMerageHead->next = Merge(pHead1, pHead2->next);
        }
        return pMerageHead;

    }
};

非递归：

class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)
    {
        if (pHead1 == NULL)
            return pHead2;
        if (pHead2 == NULL)
            return pHead1;

        ListNode* pTail = NULL;
        ListNode* pNewNode = NULL;

        //确定头结点的位置
        if (pHead1->val <= pHead2->val)
        {
            pNewNode = pHead1;
            pTail = pNewNode;
            pHead1 = pHead1->next;
        }
        else
        {
            pNewNode = pHead2;
            pTail = pNewNode;
            pHead2 = pHead2->next;
        }

        while (pHead1&&pHead2)
        {
            if (pHead1->val <= pHead2->val)
            {
                pTail->next = pHead1;
                pHead1 = pHead1->next;
            }
            else
            {
                pTail->next = pHead2;
                pHead2 = pHead2->next;
            }
            pTail = pTail->next;//尾部向后走一个
        }

        //一个链表走到末尾，把另一个剩下的连接上去
        if (pHead1)
            pTail->next = pHead1;
        if (pHead2)
            pTail->next = pHead2;

        return pNewNode;
    }
};