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206. Reverse Linked List

Linked List Recursion

Problem - Reverse Linked List

Easy

Given the head of a singly linked list, reverse the list, and return the reversed list.

 

Example 1:

Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

Example 2:

Input: head = [1,2]
Output: [2,1]

Example 3:

Input: head = []
Output: []

 

Constraints:

  • The number of nodes in the list is the range [0, 5000].
  • -5000 <= Node.val <= 5000

 

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

Solutions

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# 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: Optional[ListNode]) -> Optional[ListNode]:
        dummy = ListNode()
        current = head
        while current:
            next_node = current.next
            current.next = dummy.next
            dummy.next = current
            current = next_node
        return dummy.next

Submission Stats:

  • Runtime: 0 ms (100.00%)
  • Memory: 18.8 MB (38.13%)