Leetcode #206
Problem
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?
Optimal Solution:
# 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]:
curr = head
prev = None
while curr:
temp = curr.next
curr.next = prev
prev = curr
curr = temp
return prev