Merge Sorted Lists

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def mergeTwoLists(self, list1, list2):
        """
        :type list1: Optional[ListNode]
        :type list2: Optional[ListNode]
        :rtype: Optional[ListNode]
        """
        out = list3 = ListNode(0)
        while list1 and list2:
            if list1.val<list2.val:
                list3.next = list1
                list1 = list1.next
            else:
                list3.next = list2
                list2 = list2.next
            list3 = list3.next
        list3.next = list1 or list2
        return out.next