將兩個有序連結串列合併為一個新的有序連結串列並返回。新連結串列是通過拼接給定的兩個連結串列的所有節點組成的。 

示例：

輸入：1->2->4, 1->3->4
輸出：1->1->2->3->4->4

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

class Solution:
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        dummy = ListNode(-1)
        dummy.next = l2#將l1插入l2中
        temp = dummy
        temp1, temp2 = l1, l2
        
        while temp2 != None and temp1 != None:#當l1和l2都非空的時候
            if temp2.val > temp1.val:
                temp.next = temp1
                temp1 = temp1.next
                temp = temp.next
                temp.next =temp2
            else:
                temp = temp.next
                temp2 = temp2.next
        if temp2 == None:
            temp.next = temp1
        
        
        return dummy.next