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?
Solution
Approach 1: Iterative
def reverse_list(head: ListNode) -> ListNode:
prev = None
current = head
while current:
next = current.next
current.next = prev
prev = current
current = next
return prev
https://youtu.be/LcKcMDP6WwU?embedable=true
Space complexity: O(1) Time complexity: O(n)
Approach 2: Recursive
def reverse_list(head: ListNode) -> ListNode:
if not head or not head.next:
return head
new_head = reverse_list(head.next)
head.next.next = head
head.next = None
return new_head
Space complexity: O(n) Time complexity: O(n)