Swapping pairs in a linked list in Python, one link disappears?

问题

I have been learning about linked lists, and implementing one in python has been easier than I expected. However, when it has come to solving the problem of "swapping pairs in a linked list", for some reason my second link disappears during the swap. I've been staring at this for ages and trying different solutions I found online. They all get the same result which suggests my problem is with the implementation of the list itself. Or I've made a silly error somewhere that I can't see! I would be grateful for a fresh pair of eyes. What have I done wrong?

class Node:
    def __init__(self, val):
        self.value = val
        self.next = None

class LinkedList:
    def __init__(self, data):
        self.head = Node(data)

    def printList(self, head):
        while head:
            print("->" , head.value)
            head = head.next;

    def swapPairsR(self, node): # recursive
        if node is None or node.next is None:
            return node
        ptrOne = node
        ptrTwo = node.next
        nextPtrTwo = ptrTwo.next

        # swap the pointers here at at the rec call
        ptrTwo.next = node
        newNode = ptrTwo

        ptrOne.next = self.swapPairsR(nextPtrTwo)
        return newNode

    def swapPairsI(self, head): # iterative
        prev = Node(0)
        prev.next = head
        temp = prev

        while temp.next and temp.next.next:
            ptrOne = temp.next
            ptrTwo = temp.next.next

            # change the pointers to the swapped pointers
            temp.next = ptrTwo
            ptrOne.next = ptrTwo.next
            ptrTwo.next = ptrOne
            temp = temp.next.next

        return prev.next

thisLList = LinkedList(1)
thisLList.head.next = Node(2)
thisLList.head.next.next = Node(3)
thisLList.head.next.next.next = Node(4)
thisLList.head.next.next.next.next = Node(5)
thisLList.printList(thisLList.head)
print("--------------")
thisLList.swapPairsI(thisLList.head)
thisLList.printList(thisLList.head)

Edit: my output:

-> 1
-> 2
-> 3
-> 4
-> 5
--------------
-> 1
-> 4
-> 3
-> 5

回答1:

Your swapPairsI function is returning the new head of your linked list. You need to update it accordingly:

thisLList.head = thisLList.swapPairsI(thisLList.head)

---

Or better yet, you should change your swapPairsI function so that it doesn't have to take a node as parameter:

def swapPairsI(self): # iterative
    prev = Node(0)
    prev.next = self.head
    temp = prev

    while temp.next and temp.next.next:
        ptrOne = temp.next
        ptrTwo = temp.next.next

        # change the pointers to the swapped pointers
        temp.next = ptrTwo
        ptrOne.next = ptrTwo.next
        ptrTwo.next = ptrOne
        temp = temp.next.next

    self.head = prev.next

In which case, you can simply call:

thisLList.swapPairsI()

来源：https://stackoverflow.com/questions/46648779/swapping-pairs-in-a-linked-list-in-python-one-link-disappears