描述 输入两个单调递增的链表，输出两个链表合成后的链表，当然我们需要合成后的链表满足单调不减规则。

  1. 把结果用列表存放再返回
  2. 用递归方法，不断进行比较合并返回
class Solution:
    # 返回合并后列表
    def Merge(self, pHead1, pHead2):
        # write code here
        if not pHead1:
            return pHead2
        if not pHead2:
            return pHead1
        if pHead1.val <= pHead2.val:
            pMerge = pHead1
            pMerge.next = self.Merge(pHead1.next, pHead2)
        else:
            pMerge = pHead2
            pMerge.next = self.Merge(pHead1, pHead2.next)
        return pMerge