方法一：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def mergeTwoLists(self, l1, l2): #這裡的l1 l2都是連結串列形式，所以沒有索引沒有長度
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        
        p = ListNode(0) #先定義一個節點（頭結點）
        head = p 
        
        while l1 != None and l2 != None:
            if l1.val <= l2.val:
                p.next = l1
                l1 = l1.next
            else:
                p.next = l2
                l2 = l2.next
            p = p.next    
            
        if l1 != None:
            p.next = l1
        elif l2 != None:
            p.next = l2
        
        return head.next     
 

方法二：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def mergeTwoLists(self, l1, l2): #這裡的l1 l2都是連結串列形式，所以沒有索引沒有長度
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        
	if l1==None and l2==None: 
		return None 
	if l1==None:
	 	return l2 
	 if l2==None: 
		return l1 
	if l1.val<=l2.val: 
		l1.next=self.mergeTwoLists(l1.next,l2) 
		return l1 
	else:
		l2.next=self.mergeTwoLists(l1,l2.next) 
		return l2