合并两个排序的链表

class Solution:
    # 返回合并后列表
    def Merge(self, pHead1, pHead2):

        if pHead1 == None:
            return pHead2
        if pHead2 == None:
            return pHead1

        # 得到最小的一个头结点
        newHead = pHead1 if pHead1.val < pHead2.val else pHead2
        # 链表1上的指针 和 链表2上的指针
        pTmp1 = pHead1
        pTmp2 = pHead2
        if newHead == pTmp1:
            pTmp1 = pTmp1.next
        if newHead == pTmp2:
            pTmp2 = pTmp2.next

        # 前面的指针
        previousPointer = newHead

        # 链表1 和 链表2 都不为空的时候，开始合并
        while pTmp1 and pTmp2:
            # 找出最小值，放在previousPointer指针后面
            if pTmp1.val < pTmp2.val:
                previousPointer.next = pTmp1
                previousPointer = pTmp1
                pTmp1 = pTmp1.next
            else:
                previousPointer.next = pTmp2
                previousPointer = pTmp2
                pTmp2 = pTmp2.next

        if pTmp1 == None:
            previousPointer.next = pTmp2
        if pTmp2 == None:
            previousPointer.next = pTmp1

        return newHead