Linked Lists Gotchas

Linked lists in Python are strange concept. Here are some tips, tricks, and pitfalls.

1. Once you traverse it, say to print it, you have lost your ability to go back to the beginning. In other words, you can only print it once. You can not print it ever again. Unless you plan ahead to save the head.

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

class Solution(object):
   
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
       
        dummy_head = tail = ListNode
       
        dummy_l1_head = l1
        dummy_l2_head = l2
       
        # merge l1 and l2 into tail
        while l1 and l2:
            if l1.val < l2.val:
                tail.next, l1 = l1, l1.next
            else:
                tail.next, l2 = l2, l2.next
            tail = tail.next
           
        tail.next = l1 or l2
       
        tail = dummy_head
        l1 = dummy_l1_head
        l2 = dummy_l2_head

        # append l2 to l1
        tail.next = l1
        while tail.next:
            tail = tail.next
        tail.next = l2
       
        return (dummy_head.next)