题目描述
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。

# -*- 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
        # 运行时间：23ms  占用内存：5732k
        if pHead1 is None:
            return pHead2
        if pHead2 is None:
            return pHead1
        
        previous = pHead1 if pHead1.val < pHead2.val else pHead2

        temp1 = pHead1
        temp2 = pHead2

        if previous == pHead1:
            temp1 = temp1.next
        else:
            temp2 = temp2.next

        while temp1 and temp2:
            if temp1.val < temp2.val:
                previous.next = temp1
                previous = temp1
                temp1 = temp1.next
            else:
                previous.next = temp2
                previous = temp2
                temp2 = temp2.next

        # if temp1:
        #     previous.next = temp1
        # else:
        #     previous.next = temp2
        previous.next = temp1 if temp1 else temp2
        return pHead1 if pHead1.val < pHead2.val else pHead2