Problem State
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:
1. Recursion
class Solution {
public ListNode reverseList(ListNode head) {
if(head == null || head.next == null){
return head;
}
ListNode reversedHead = reverseList(head.next);
head.next.next = head;
head.next = null;
head = reversedHead;
return head;
}
}