21_MergeTwoSortedLists.py

# 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]
        """
        dummy = ListNode()
        tail = dummy

        # move through both lists and add the lesser value to the tail
        # of our new list
        
        while list1 and list2:
            if list1.val <= list2.val:
                tail.next = list1
                list1 = list1.next
            else:
                tail.next = list2
                list2 = list2.next
            tail = tail.next

        # at this point we've reached the end of one of the lists
        # so just tack the remaining other list on at the end
        if list1:
            tail.next = list1
        else:
            tail.next = list2

        return dummy.next