How to reverse a Linked List⌗

206. Reverse Linked List⌗

Difficulty: Easy

Reverse a singly linked list.

Example:

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

Follow up:

A linked list can be reversed either iteratively or recursively. Could you implement both?

Solution⌗

Language: Python3

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
​
class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        prev = None
        while head:
            current = head
            head = head.next
            current.next = prev
            prev = current
        return prev
    
# basic idea of how to iteratively reverse a linked list is:
# each time iterate the head, let head point to the prev one

Reverse Linkedlist recursively⌗ 

class Solution:
    def reverseList(self, head):
    """
    :type head: ListNode
    rtype: ListNode
    """
    cur, prev = head, None
    while cur:
        cur.next, prev, cur = prev, cur, cur.next
    return prev

Conclusion⌗

Iterative and Recursively difference: Iterative continuously calles functions while recursively calles itself