How to reverse the linked list with python

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[title]

Reverses a single linked list.

Example:

Enter: 1-> 2-> 3-> 4-> 5-> NULL

Output: 5-> 4-> 3-> 2-> 1-> NULL

Advanced:

You can iterate or recursively reverse the linked list. Can you solve the problem in two ways?

[ideas]

Use three pointers p, Q, r to point to three adjacent nodes, where p.next is qmenq.next is r.

Modify the pointer to Q and move the three pointers p, Q and r, that is, q.next = pforce p = Q, Q = r r = r.next. Continuous circulation, and pay attention to modify head.next and head, the list can be flipped.

[code]

Python version

# Definition for singly-linked list.

# class ListNode:

# def _ _ init__ (self, val=0, next=None):

# self.val = val

# self.next = next

Class Solution:

Def reverseList (self, head: ListNode)-> ListNode:

# head is empty

If not head:

Return head

# p, Q, r are three adjacent nodes

# q.next points to p, while three pointers move backward

P, Q, r = head, head.next, head

While q:

R = q.next

Q.next = p

P = Q

Q = r

# modify head.next and head

Head.next = None

Head = p

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