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。

* solution1: Recursion
```python3
# -*- coding:utf-8 -*-
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    # 返回合并后列表
    def Merge(self, pHead1, pHead2):
        # write code here
        if not pHead1 or not pHead2:
            return pHead1 or pHead2
        elif pHead1.val < pHead2.val:
            pHead1.next = self.Merge(pHead1.next,pHead2)
            return pHead1
        else:
            pHead2.next = self.Merge(pHead1,pHead2.next)
            return pHead2
```
* solution2: Dummy + Iteratively
```python3
# -*- coding:utf-8 -*-
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    # 返回合并后列表
    def Merge(self, pHead1, pHead2):
        # write code here
        dummy =
```