CODEWITHSUNDEEP
pythonpython-3.xlinked-listreverse
Sanya
Sanya

Reputation: 73

Reverse linked list - how to remove "None" which is coming at the end (of output)

I am solving this problem on leetcode - Reverse linked list

My code is running fine I believe but I am receiving "None" in the output. How can I remove this?

This is how output is coming for the input [1,2,3,4,5]:

Output: [5,4,3,2,1,None]

Here is the code I tried:

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:

        if head is None: return None

        if head.next is None: return head

        cur = head
        prev = ListNode(None)
        temp = head.next

        while temp:
            cur.next = prev
            prev = cur
            cur = temp
            temp = temp.next

        if temp is None:
            cur.next = prev

        head = cur
        return head

Upvotes: 1

Views: 201

Answers (2)

coder
coder

Reputation: 12972

As a complementary answer, you are using too many useless if statements that you could avoid. Similarly to your answer, your could reduce it simply to the following:

def reverseList(self, head: ListNode) -> ListNode:
prev = None

while head:
    temp  = head.next
    head.next = prev
    prev = head
    head = temp

return prev

Upvotes: 1

abc
abc

Reputation: 11929

With prev = ListNode(None) you create a new Node with value = None and next Node None.
So, in your example Node 1 does not have None as self.next, but a Node with None as self.val.

prev = None

should solve the issue.

Upvotes: 1

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