采用递归和非递归方式实现

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param pHead1 ListNode类 
# @param pHead2 ListNode类 
# @return ListNode类

非递归方式

class Solution:
    def Merge(self , pHead1: ListNode, pHead2: ListNode) -> ListNode:
        # write code here
        # 非递归方式
        # 创建一个新链表，使用虚拟节点dummy作为哨兵节点指向新列表头部
        cur = dummy = ListNode(0) 
        while pHead1 and pHead2:
            if pHead1.val < pHead2.val:
                cur.next = pHead1
                pHead1 = pHead1.next
            else:
                cur.next = pHead2
                pHead2 = pHead2.next
            cur = cur.next

递归方式

class Solution:
    def Merge(self , pHead1: ListNode, pHead2: ListNode) -> ListNode:
        # write code here
        # 递归方式
        if not pHead1 or not pHead2:
            return pHead1 or pHead2 
        if pHead1.val < pHead2.val:
            pHead1.next = self.Merge(pHead1.next, pHead2)
            return pHead1 
        else:
            pHead2.next = self.Merge(pHead1, pHead2.next)
            return pHead2